Lesbian? Please Help.

Hi everyone. I'm really sorry this is going to be a long post, but I didn't know where else to turn but an online support community. I hope you can help me.

I feel like I am trapped in an impossible situation and I don't know what to do. I am a 21 year old female and I have been in a happy relationship with a man for 2.5 years. We live together in his university town, and were very content with our relationship until a few months ago.

I have always identified as pansexual, as up until now, gender did not matter to me. My boyfriend knew I identified as such, and had no problem with it. A few months ago however, I noticed a shift in my feelings. Women became more sexually appealing to me, and my sex drive declined dramatically. My partner has some sexual problems which I won't go into detail with, but it wasn't an issue that we were no longer regularly sleeping together.

I tried to ignore my feelings, and about a month passed and we moved into our new property together (we were in shared student housing before, but now it's just the two of us). Things were going really well, but I had some problems with my family and my younger sister tried to commit suicide. She went to hospital, and is now in an adolescent hospital, and I began to feel more and more alienated from my partner, despite the fact I still love him (or at least feel like I do). To shorten this part of the story, my parents had a family holiday planned but it had to be cancelled because of my sister's situation, but my parents couldn't get a refund so my partner and I were invited to go instead, and we did.

I decided (because I am great at picking my moments), to tell my partner that I had begun to find women more attractive than men. He was really upset, and worried about our future together. He is the most supportive, kind-hearted man in the world, and I really do love him; if I left him, he wouldn't be able to afford to live where we live now, and he'd have to leave university which he loves. He has a really bad relationship with his family so he would have no where to go but home, and be unhappy, and he would also have to stop going to the gym which has really helped him overcome his depression.

I don't want to ruin his life by leaving. It would totally break his heart - he always tells me he loves me, and that I'm his soul mate, and I used to be able to say it back and mean it, but recently it's been harder and harder. While we were away on holiday, his sexual problem went away on the one night we were sexually active, and this was the first time in 2.5 years that we had 'normal' sex - whatever that is. I feel guilty even thinking about leaving him because it would totally destroy him, and me, but it's not fair for me to keep thinking about women.

My biggest fear would be ending this relationship with my boyfriend and best friend, and realising a year or a few months down the line that I've made a massive mistake. I am so confused, as I love my boyfriend but am just totally flatlining when it comes to sex appeal, and often think about how happy I would be with a woman.

I have never been with a woman, so I have no experience, but from a young age I have always been attracted to both sexes. It is only recently that I feel far more attracted to women.

Any advice here would be so appreciated. My boyfriend said he can give me time to think about how I feel and what I want, but I know he thinks it's his fault. He's being super nice to me, and sending me cute messages and pictures of us together when I am away at work, and it makes me feel so guilty. I'm struggling to sleep at night because I feel that bad! Can I really be a lesbian if I love my boyfriend? What should I do?