I have talked to both of them about this situation and how its hard for me to be stuck in the middle of. They both agree to back off, but then they go at it again the next week. I've tried to fix the situation many times, but its always the same story.
I'm not sure where to go from hear. I feel like I cant talk about my best friend to my boyfriend, or talk about him to her. Please help me. I cant take this much longer.
A: Aly, unfortunately your situation is not an uncommon one. However, it is extremely immature and spiteful of your boyfriend and best friend to be doing this, especially if they are fighting over their relationship from 4+ years ago.
You say that you have tried to talk to both of them, and I believe that. Don't try to analyze them or talk to them any further about their hatred. There is no point in trying to figure out why they hate each other so much, because it's just going to be petty and immature.
The best that you can do is hang out with them individually, FAR away from the other person. If whomever you're hanging out with starts trashing the other person, tell them "Zanne (or Michael), I'd really appreciate it if you'd not talk about my boyfriend (or my best friend) that way. I understand that you don't like her (or him) and I get that, but I'm not going to be in the middle of it any more. You're hanging out with me right now." BE FIRM. If they continue to act immaturely, then tell whoever it is that you will see them tomorrow or something and leave.
You say that they are your only friends. I don't believe this is necessarily true, but regardless of anything, you need to find someone whom you can discuss your issues with Zanne and Michael with. Someone who is easily accessible, lives by you, and who you can talk to in person any time you want. For many reasons, boyfriends and best friends frustrate us, but you have learned that venting to either of them is disastrous. If you are close with anyone else--parents, other relatives, other friends--that'll help a lot. If not, possibly look into finding a counselor at home who you can see every week or so.
Best of luck, and please, don't be afraid to stand up to your friends if they continue this BS! :)