It’s an interesting question that pops up regularly. You have only two choices. “It’s their fault” or “it’s yours”. Your answer to this question will say a lot about your maturity.
When I hear “it’s their fault”, the person usually tells a wonderful story of everything the other person(s) did to sabotage your efforts or hurt your feelings. It does not have to involve an immediate situation. An example of this is when someone is struggling in life and they blame their parents for this or that.
This answer, “it’s their fault”, leads to repeated failures because the person never adjusts their approach or thinking. This answer is equivalent to saying that it wasn’t my fault. No learning occurs. And this answer makes you sound like a spoiled child who simply didn’t get their way.
Your other choice is to say that it is my fault. I could have done something differently. Just these words alone imply introspection. Reviewing what took place and trying to determine your actions that may have led to an undesirable outcome. Especially when it comes to relationships, both parties played some role in the outcome.
A beloved mentor of mine used to tell me this all of the time — to first look inside at myself. And after this self examination as to what I could have done differently, he also stressed to me that if I truly felt I was not at fault that he found it effective to accept part of the blame publically to move a relationship forward. To always try to be the bigger person.
For ego is what damages relationships and many people cannot see when it gets in their way. This is much harder to do in practice and if you can, it is a great sign of maturity. For our lives are so short and relationships are important to grow and share that we must always try to keep them stable.
Of course, as a last resort, you could just leave the relationship totally. Sometimes it’s the only thing that can be done. And just as effective if the relationship that is contentious will not give you any opportunity to make progress on your goals.