Your therapist is correct - this is her issue. However there are a few things you can potentially do on your end too but it involves sharing your feelings and putting stronger boundaries in place - both are difficult.
My wife constantly was bringing up items like this - sometimes from 20 years ago. I did my therapy work and explained to her my feelings something like this: people make mistakes, I am not perfect. It makes me feel bad that you are continuing to bring it up my mistakes after I have genuinely apologized. It makes me feel that I am not safe emotionally and I can’t make mistakes with you and grow as a person. When you continue to bring up my mistakes from the past it makes me feel … etc. … what can we do to change this situation? I will make mistakes, and I want to feel emotionally safe around you and grow towards you - not be afraid of making a mistake and start withdrawing from you …. (Then listen with empathy and validate her feelings - this step is crucial - use the SET methodology)
Then set a boundary: I no longer want to feel this way. I want to feel respected, and I want to feel I am allowed to make mistakes and learn from them, and that each mistake I make will not be brought up for the rest of my life. I don’t want to live in a world where all of my mistakes will be perpetually brought up, and definitely not from someone from someone who I need to be my biggest cheerleader and encourager. I want to live with a partner that knows I am not perfect and I will make mistakes, and won’t be punished for them forever. I want to live and grow into in the future - not continuously relive the dark parts of my past. I am unable to change how you feel about “x.” I feel I have tried everything I can, and I can’t change the past. How you feel about “X” I realize now is out of my capability to change. I need to let go of trying to control how you feel. I can only control how I feel. So - if you choose to bring it up again, I will choose to … (pick something….change the subject, stop the conversation and leave the room for 5 mins to decide how we will proceed to come back and talk… etc). I can appreciate that what I did hurt you deeply, and I acknowledge my role and have taken accountability. Now for my feelings I need to move forward, and not discuss it again. (It is important that the boundary consequence is proportional, and something 100% in your control to implement and it doesn’t require her participation at all.)
Optional: …I feel that this is hard for you to let go of these feelings, and I feel you are struggling from the hurt I caused…. I care about you and I don’t want you to feel this way. I will 100% support you in helping you move forward too so we can be closer. If you feel it is too hard to let go of this hurt and not bring it up again I am willing to support you in talking this out with someone more qualified than me on how to manage these difficult feelings…etc. (counselor etc..)
Hope this prompts some ideas…