I need help... 😞

I'm not proud to admit this but I'd be perfectly fine being in an open relationship. I have a boyfriend, I've been loyal to him but I have done some thinking and figured out why my relationships never last-- it's not my partner, the problem is me. I tend to get bored easily with relationships once the spark/honeymoon phase wears off. I'll still have feelings for my partner but I'd start to want more and other things... as of yet, that has not happened with my current boyfriend. I would like to speak to him about this but I honestly don't know if I should because he seems to get mad when guys flirt with me but always talks of having threesomes and such.

I guess I just need advice/opinions?