Has anyone made a toxic relationship work and are now healthy and happy together?

I do believe that a toxic relationship can return healthy but not without great conscious efforts and emotional work. Forgiveness, self-awareness, and a true desire for that end goal. He would need to unlearn harmful behaviours and you will likely need to heal from your experiences as well as you may now have triggers and some degree of trust issues (outside of infidelity) etc.
Couples therapy could be beneficial because both of you will need to work through this together xxx

My late husband and I had a toxic relationship, but every time we got therapy for specific issues, they always immediately and drastically improved, even ones I was certain he was doing intentionally (I was wrong). He weren’t non-toxic when he died, but I believe we could have gotten there.

I’ll be honest to improve a toxic relationship both parties have to want change and improvement.
Couples therapy, someone to hold you both accountable etc.
it is possible.