Feeling sick at heart and now self harming again

This might be quite long as I try to explain it so brace yourself:

About a year and a half ago it was always just me and my bestfriend Hannah. We are both very boyish so I thought of her almost like a brother. I had other friends, and no-one in my class at school hated me (hopefully!) so life was great.
Later on I slipped into a downwards spiral of depression where I absolutely no self confidence and I always thought myself as not good enough to live in the world. I constantly contemplated suicide and ways to get out of the shithole that had become my life.

Things did improve when I dropped a few friends who had only ever made me feel terrible about myself and Hannah and I sometimes floated between friend groups but always stayed together.

About a year ago now we became increasingly friendly with two girls named Charli and Mya. These two always walked around together just like me and Hannah did at the time.

As we became more friendly, we became a group of four. All four of us walked around together and it was great for a while but soon Charli began to get inbetween me and Hannah. It was small things at first, such as Charli simply asking her to walk to her locker with her or asking her to come up to the line in the canteen to buy her lunch with her. But soon Charli was whisking Hannah away on weekends out with just the two of them. Hannah was reluctant at first but soon she became even more fond of Charli. They began to do things such as hold hands when they were walking around and cuddling up together on sleepovers, and just generally treating eachother like girlfriends. (I'm not sure what either of their sexual orientations are).

Me and Mya were both very put out and were left with nowhere to go. I turned back to feeling sad everyday as I felt as if I had no one left. But Mya and I were left alone together so much that we soon became very close friends, but of course me and Hannah still stayed friends also. (I should probably add now that both me and Mya are bisexual.)

We became increasingly close and my world started to light up again. She treated me like Charli treated Hannah. Like I was her girlfriend. She made me feel special as she was never affectionate with anyone except for me. Recently (a month ago now) she slept over at my house and we were messing about on ******. We were 'pretending' to everyone that we were a couple and many people believed us. There was one girl who said, "Prove it, kiss." when we told her we were together. I looked over to Mya and laughed but she kissed me for a few seconds. I felt butterflies in my head and in my stomach and I was seeing everything in purple and my heart was beating so fast it was crazy. She brushed it off saying, "This doesn't mess things up with us right?" and of course I nodded and said it was fine.

About a week later we gradually fell apart a bit. She started to call me names like cunt, pussy and dickhead when I spoke to her. She apologized later on in the week for this though and when I told her I just didn't want to lose her she hugged me close. I thought it would all be back to normal again as we went out to town that weekend and it was all fine. But once we got back to school her and Charli began to leave Hannah and me out a bit, they seemed to have many in jokes and it looked like they had met up over the weekend without telling Hannah and me. This continued for another week until Charli and Mya started getting affectionate, and me and Hannah were left out in the cold again.

Its been like this for a few weeks now and I feel a physical pain in my heart when I see them both together. She hurting me so much and now shes making me worse and worse every single day. I tried talking to her about it and she just said "Charli is my bestfriend." as an explanation. It hurts so badly and I don't know what to do to get her back? I'm beginning to hate her for making me feel this way but every time I see her I want to throw my arms around her and let everything be like it was. I'm turning back to self harm and we have to wear skorts (skirt/short thingy) for P.E now and people are starting to notice scars on my legs and I don't want teachers to make me go to the school counsellors. When I see them acting like a couple at break or lunch time I literally get up and leave, then I go to the bathroom and take down my tights and self harm on my lower leg or thigh. I can't seem to help it, its the only way I can let it all out.

Help me? I've fallen so far and I thought I wouldn't turn back to self harm again.

Sorry if this is so long but please some helpful comments?