There is one supporter relationship in my life that was rebuilt when my trust was completely gone. It was not someone I love, but actually someone in a helping professional role with me. (Not a therapist.) I was so hurt and so devastated by something they did that I fired them. Angrily.
They responded by saying, "you are right, I dropped the ball. I screwed up. I'm still on your side. I support your right to fire me. I still want to help, but if you thought this through, and this is what you want to do, I will support your decision." It was a response that stopped me in my tracks.
I had put my future in their hands and it was a pretty major thing they did. Not intentional, but she knew how much I needed her to come through and she just spaced it, and I was hurt. Because it wasn't malicious and because she owned her humanity and her mistake, I hung in. It did take time and for awhile, my lack of trust was hard for her - like she would say, "I am working for you, I know you don't trust me, and that's ok, I just want to show you that I am working for you. I got this."
I didn't feel trusting of her for a long time, but I kept taking small risks and she kept handling it well - not perfectly, but her intent was pretty clear. She was pretty enduring in working in through with me.
After awhile, I did trust her again. She couldn't tell - she would say, "I hope you can keep trying to trust me." My trust was there in my heart, but I had a hard time knowing what trust even looked like.
By the end of what she was helping me with, I did trust her really deeply.
It wasn't the same trust as before, but in this experience, it was almost just as good and it was so worth it. I never did trust that she wouldn't drop the ball again... I trusted that she was human, and I did come to trust so much more deeply that I did initially that she really was on my side.
I had some therapy in the past before this, but it was bad therapy that decreased my trust in helping professionals in huge ways. In this non-therapy setting, I learned a lot about trust. There were times this person had to trust me or it wouldn't have worked.
Working on my own healing and recovery was KEY to the supporter being able to rebuild trust with me. If I had not been trying to do relationships different and heal from the past, there would have been nothing she could have done to rebuild trust with me.
Her combination of humility with the strength to not also be a doormat really helped too. It was actually humbling for me and rather life changing.
There are a couple of relationships I have not been able to rebuild trust - and it is partly because they never tried to change or own their part. Sometimes it's hard to own my own part - rebuilding trust is hard and humbling to do!
Therapy can help someone work through forgiveness. Someone once told me that not forgiving is like drinking posion and hoping it hurts someone else. I think that is true for me (maybe not for others, it probably varies a lot). I find it hard to forgive, and yet it's painful to stay scared and angry. I don't think forgiving always means reconciliation or being able to trust the person again. Sometimes it can't mean that or shouldn't mean that.