Any advise for ending a long term relationship?

Any advise on how I [25/F] end a 4 year long relationship with my boyfriend [37/M]? I have never had to leave a relationship that was longer then just a couple of months...this has been my longest relationship and we have lived together for 3 of these 4 years...and due to issues in the relationship that have caused resentment I haven't been able to move past, I have decided that leaving the relationship will be best for both of us in the long run. I cant be with someone I have such resentment for and its not fair for him to have something being held over his head constantly. He loves me, and I still care about him and his feelings, but I just know it wont work in the long term....any advise from anyone who has been in the same situation?

** tl;dr**: How to gracefully leave a 4 year long relationship - we live together.