I'm going to cut off my best friend because I think he used me
Well, could you explain to me why you believe your best friend used you?
I've found out that he used to get involved with several old friends of mine
How does that harm or use you, though? Is that really enough reason to give up your best friend?
I just feel bad, we aren't that close now anyway.
If you consider him your best friend then you shouldn't let something like this get in the way, although it must be hard for you, you should give him another chance. I doubt he knowingly used you.