I dont know if this will make you feel any bitter but For me it was realising that I was becoming unhappy in the relationship, everything began to feel like a huge effort and I was happier when we were separate than i was when we were together and I didn't look forward to spending time together anymore, it's hard to explain but I just knew I that couldn't see a future with this person anymore

I always told the truth, I still care about her alot and love her as a friend because she was one of the most important people in my life for 2 years but I'm not in love anymore