I'm going to break up with my boyfriend because of his political beliefs.
Why are you breaking up with him based on his political beliefs?
Because they're too extreme and they've consumed him.
I think that there are plenty of successful relationships that have different political beliefs. You should not break up based solely on those reasons.
He can't stop talking about his beliefs and he's changed too much.
Talk to your boyfriend and tell him how his political beliefs are affecting you and your relationship. When he understands how serious this is, he may be willing to change to save the relationship.