How do you know when it's time to end a relationship?

I once posted in this community about an incident with the HVAC repair guy that nearly upended our relationship. (TW b/c there is a description of self harm) We stuck it out since then. Went to couples therapy, etc. It helped for a while, but at some point it ran its course.

Today, we're worse off than we were before therapy. I'm so exhausted. I feel tense and like I have to walk on eggshells around her. I really thought she was the one, and now I just don't know what to think anymore.

How do you know when it's time for things to end? I'm worried it'll be a terrible mistake to end things, but at the same time, it might be for the best.