Confused

So, let me start off by introducing a little about me. I'm 19, bisexual, and am currently in a heterosexual relationship of four and a half years. I guess I knew I liked girls when I was younger, I was always intrigued by them. It never really became increasingly noticeable or sexual, though, until I was about 16 or 17. My boyfriend noticed I would check out girls as they walked by. He, as any typical teenage boy does, found this extremely attractive. He'd always call me bi-curious and I'd be like "Oh, whatever!" and sometimes get rather embarrassed. My boyfriend convinced me that I needed to befriend a girl I went to school with who was openly bi, mostly leaning towards girls. And, so I did.
We became close friends very fast. She and I had everything in common, and always had a lot of fun. She knew I was bi, and even helped me to come out. She thought it was cute. My senior year (since my boyfriend was a year ahead and already graduated) I took her to prom with me as my date. Throughout our friendship, I felt for her in a different way. Sometimes, I felt as if it was a crush. At one point, when she was dating this guy, I would get so jealous when he was around I felt sick. I didn't want to tell her though because I was sure she did not have any feelings for me. We would always playfully joke and flirt with each other about either innocent or sexual things. I always felt like I was overstepping my boundaries if I was to say that, well, maybe, I wasn't kidding.
Once summer hit after we graduated, she confessed that she wasn't sure if she was somewhat falling for me, or if she was confusing a good friendship with a crush.
Although she admitted that, I wasn't exactly sure if we were on the same page. One night, a couple months back, she stayed the night at my apartment (I live with my boyfriend). We both got really drunk, and she kept joking to my boyfriend saying "Oh, I'm gonna steal your girl! Is that okay?" My boyfriend told her he didn't care. I thought she was just joking until she came and sat next to me and kept asking him if it was really okay. I started getting crazy butterflies and almost felt nauseous with nervousness. I put my head down into my lap to avoid looking her in the eyes. Then I heard her say "As soon as she lifts her head, I'm going to kiss her, okay?"
I don't remember if I lifted my head myself, if I didn't really understand what was going on, or if she lifted my head, but the next thing I knew we were kissing. It was amazing. My boyfriend thought it was the hottest thing in the world, but went back into the other room to play his video games because that was probably the end of it.
My best friend and I giggled and went on with the night. My heart was pounding. Later, I went into the kitchen to grab a drink. She came in to the kitchen and was all "Are you sure this is okay?" I responded with a yes, and she came close to me. She was like "So I can kiss you like this and you won't get mad?" and kissed me again. This time we were standing up, and both pressed against each other. I had my hand on her hips and she had hers around my neck. Again, we kind of smiled and giggled, and went back to the couch. Our giggles that time, though, sounded more seductive.
On the couch, she kept telling me she needed to sit way over on the other side or else she wouldn't be able to control herself. I was getting nervous because I knew this wasn't the end of it. I tried to change the subject to something on the TV. Then, she asked if she could come over to the side of the couch I was on. I said yes. She started kissing me so passionately and eventually got on top of me. She bit my lips and ears. She was grinding and dry-humping me. I had my hands just above her breasts and she told me not to censor myself. I then placed my hands on her breasts and played with them. She slid her hand down to my pussy, and began to rub it through my pants. I was beginning to moan at that point and, since my boyfriend had friends over, I didn't want them to hear us or walk out and see us. So, I told her it was probably best if we went into my bedroom.
Just as we were making our way in there, my boyfriend saw me clearing the bed. He knew what was going on.
She shut the door, and started to passionately kiss me again. I heard my boyfriend kicking all his friends out. She start kissing lower and lower on my body and asked if this was okay what must of been a million times. I kept saying yes through what breath I had left. She took off my pants and underwear in one quick motion, spread my legs, and I think you can imagine the rest. My boyfriend walked in, watched us, and when I tried to play with him and get him involved, he was literally so in shock he couldn't get hard. She went down on me for a long, long time. It was the best I've ever had. I think about it every day. When I had finished multiple times, she came up towards me and we kissed. I played with her through her pants and her moans were so sexy. We wanted to take it further, and I was going to return the favor, but I think she got worried that my boyfriend would get mad about this, or that we'd feel awkward in the morning, so she went and laid on my couch. I just laid in my bed in shock thinking about how she made me scream.
It wasn't awkward the day after, and still isn't. My boyfriend isn't mad, she isn't mad. Everything is normal. I just know he wants to get involved next time, but I think I want to be selfish.
Since then, we've done nothing else, as much as we both wanted to. We've texted each other late at night, playing with ourselves and saying naughty things. We've told each other what we're going to do to each other, and we've recently started watching the show The L Word, and have admitted to how turned on it makes us and how much we crave lesbian sex.
Last night, I stayed at her house. I was going to leave around 10:00pm or sooner to go home and see my boyfriend for dinner. But, instead, we smoked weed, we laughed, and we watched The L Word. We were both very apparently turned on, and all I wanted was to kiss her even if nothing else. She changed the show because I think it was getting to her too much, and we just watched Family Guy. She fell asleep, and slowly I moved inch by inch to cuddle her; but I would fake being asleep so she wouldn't know I was doing it purposely. I left at 5:00am since my boyfriend didn't know I was staying the night (even though I pretty much did) and so I could be home before he woke up for work. She asked what I was doing when I grabbed my shoes, and I felt so awful just leaving her.
I don't know what the point of telling you all this was, I just wanted to get our story out. I'm starting to feel things for her that I haven't felt since my boyfriend and I were first dating. I get turned on just thinking about her body, and when my boyfriend and I have sex, I imagine I'm doing things with her instead. I'm not in love, right now it's almost just lust, but I definitely feel something for her. Something stronger than just sex. I watch her and smile at her personality and beauty.
What do I do? How do I figure out what I need to do? I love my boyfriend, I could never leave him, but I don't want to give her up. I'm also unsure of the threesome thing. I just want her to myself, regardless of how hot a threesome would be. I feel like I'm being unfaithful at times. I'm bailing on my boyfriend to see her, even though he has been kind of different recently (we're just so far along in our relationship things aren't as exciting as they were at first). On nights when he doesn't want to have sex, I will text her and we'll get off to each other. Right next to him, I tell my best friend things I would have never been brave enough to tell him about.
I don't really know what I'm seeking advice on, or what the point of this really was. Maybe I just need input.