Did you notice how you leashed out at your friend/parent/co-worker or any other human being to that matter, when you have been going fine with them for so long and nothing before had ever disrupted the relationship!
It might be so, that the problem started when you came across a person, who was quiet different than you and there were a lot of things that can cause friction but along the years you learned to keep you mouth zipped and compromise for whatever reason like maybe you needed the support back then or something alike.
But what happen is most of us are so afraid, of telling others what actions of their we didn’t like, what things irritated us, those little things so not important and yet so much bitterness just build up on them.
This bitterness will continue to grow slowly and in such a subtle manner that most of the time it won’t even feel to grow, but it is never this way and some day, a little or big issue will be the last of your yielding and compromising and it will come out in the form of leashing at that person or just letting go of the relation and never wanting to see the person face ever again!
So i will advise that it is very wise, that you speak up, in a positive manner and not a stern or harsh way, about how you feel from the start and be confident and polite about it, so that the other person knows what sets you off and how the relation could be made better.
Because the thing about bitterness is that : If kept inside, it eats you and when it comes out of its dark, dreary whole where it was kept for more time than necessary, it stings you 10 times more worse.
So contemplate over things that can make you bitter towards people and share it with them albeit in calm, polite way.