Should I be messing around with my best friend? I'm in a long term relationship.
Probably should not. What drives you to do that?
I don't think I'm ready for a long term relationship.
If you don't think you're ready for a long term relationship, then call it off.  It's wrong to cheat, even if it is to sabotage a relationship you want to get out of.  Just be honest with your partner about it not working out for you.
I don't want a divorce. I just want to mess around.
If you don't want a divorce, you should work on your marriage instead of cheating. It will lead to an unhappy marriage rather than a happy life.