How to know when to end a relationship ?

My girlfriend and I have been dating for a year. We are very different on a lot of areas which has not made the relationship always easy but the attraction was very strong. For a long time I thought the relationship had no future but I was willing to enjoy it while it lasted, which didn’t make me really happy and committed.

But then I managed to change my thoughts about her and see how our differences could be strength how it didn’t have to mean we couldn’t work out in a long term relationship. And the moment I managed to think those thoughts and the relationship was amazing and working really well.

But then, about a month ago, we had a fight. I found myself very uncomfortable and walking on eggshells around her. She was always criticizing, or I could feel her being annoyed by anything I would do. the relationship felt completely forced and fake.  When I finally spoke up to say I wasn’t feeling good, she admitted that indeed, she was focused on everything she didn’t like about me and thinking how it couldn’t work out between us.  I was not expecting this at all and felt so betrayed.  I trusted her, I trusted us and out of nowhere, when things were great, she had started doubting.

We decided to stay together and try to make it work.  We didn’t see each other for 10 days right after because we went to our families for Christmas.  Spending time away from her felt good, like I was feeling like myself again.  I wondered if I shouldn’t end the relationship, if it was right for me.  She felt the opposite, she kept telling me how much she missed me, that she was sorry she doubted and that she wanted us to continue for sure. I didn’t say anything about my doubts.

Now we are back spending time together. And she is being really nice and loving. But it’s like it’s my turn to be annoyed by anything she does.
It’s like we can’t connect at all. I don’t understand her. I don’t know what to talk about. I don’t know how to be. She feels like a stranger to me and so separated from me.  Anything she does brings me back to the thought that we are wayyyy to different and there is no future together and maybe its time to move on.

But I’m feeling stuck.  I don’t want to talk to her about it again when all we’ve been doing is talking and I don’t want to have another pointless draining conversation about how it’s not working out.  And I would like to just change my thoughts like I did before… But I don’t know how.  I wonder how things could be so well a month ago, how I was feeling so in love and ready to move in with her and just a month later its like everything has shifted and I’m thinking of ending the relationship.

When do I know when it’s time to end a relationship or try to work at it and change your thoughts about it ?