Can a relationship that's been struggling for 2+ years be fixed?

We’re into our 5th year together and the past 2–3 have been an almost constant struggle to be ok. That’s not to say there weren’t happy times, of course… But it feels like much of it has been, and continues to be spent trying to “fix” the relationship. In that time, we’ve both said and done things which, to the best of our forgiveness, have compounded issues and now further hinder our efforts.

She is more than willing to work on it as we always have. But now, I feel so utterly exhausted and consumed by this seemingly futile effort that I’m ready to call it quits. The stagnation, depression, stress, and lossing parts of myself that has resulted cannot be endured another year. (at least that’s what I tell myself)
My analogy of the matter is that we’re like a painting that’s just become way too cluttered and messy; and here we are trying to fix that mess by adding more paint. How long would you sit and try to fix a ruined painting? You could spend your whole life trying to fix a bad painting only to never create the masterpiece(s) that would have followed. At what point do you just grab a nice fresh canvas, and start a new painting?

My question, is for those with experience.
Were you ever able to fix a relationship after so much trying? Was it worth the wait and effort?
Did you think it fixed, only to find later it wasn’t, and never would be, and ended up having to walk away after all?