Relationship problems

Hi, I am a 20 year old female and I am so confused and sad that I don't know what to do anymore.
I was going out with this guy (now my ex) for 3 years. Everything was going great for the whole time we were together. We had a great relationship with alot of trust and friendship and anything else you could possible ask for in a good relationship. The problem is that I started going out a lot and partying towards the end of our relationship. I think partying brought out a side in me that wanted to have a lot of fun, drunken fun. Anyway whenever I went out, my ex (we will call him "guy A") never came out with me. I started having a lot of fun away from him and barely spending any time with him. Soon enough I broke up with him. It was a somewhat hard conclusion to reach and it was very painful and I don't know why I did it even now.
Shortly after me and "guy A" broke up I started seeing someone else, "guy B". I didn't realize it at the time how much of a rebound relationship this was. I've been going out with "guy B" for almost 6 months and things are going well. The problem is that it happens a lot that when I'm not with him I think so much about "guy A" and often times when I'm with my current boyfriend I start to cry thinking about my ex.
To make things worse me and my ex still talk. Lately we've been getting together and I even cheated on my current boyfriend with my ex. I still care so much about guy A that I cant help but call him up because I miss him so much. It hurts me so bad that I had to let go of him and that I am dating someone else now.
I don't know what to do because I still love my ex so much and I am not ready to let him go. The problem is that I cant let go of my current boyfriend because things are going well. He likes me a lot and I like him a lot too...but it will never escalate to anything more because I still think about my ex so much. I guess I am not ready to let anyone else into my life but I don't know how to let people out either.
I just don't want to have to make a decision. I know that if I stay with my current boyfriend then things will continue to go good (eventhough I have to break up with him because I cheated on him). The problem is that if things pick up again with me and my ex I don't want to break up with him again.
I don't know what to do..someone please help. I cant stop crying over this big mess that I am in.