I'm anxious because my boyfriend is amazing.
Are you happy in your relationship?
Yes, I'm very happy.
It's understandable that you're anxious when thinking about how great your boyfriend is, because you don't want to do anything to ruin the relationship. Eventually you'll become more relaxed when you realize that your boyfriend also really likes you, so there will be nothing to worry about.
I want to breakup with him because I feel he is going to end up leaving me for someone better.
That's a terrible way to think. It sounds like you have some insecurity you need to figure out because you can't handle a loving relationship. If you break up with him over this then you will keep breaking up with people for petty reasons.