YTA you’re forcing her to lie over something that you should have already talked about with your fiancé. this is your life partner and you are hiding a HUGE and deeply impacting part of your life from her. you know she loves and cares about you yet you choose to hide such a vital part of yourself. please do you, your partner, and possibly future kids a favour and go to therapy. regardless of a good life you hav built you clearly haven’t gotten past your own traumas and those can impact your spouse and children daily whether or not you realize. you owe it to yourself, your future spouse, and any future family you have to get help for yourself. your past was traumatic, acknowledge that and deal with it appropriately. you are an adult but you are suffering from childhood/teenage traumas daily. you and she deserve better so make moves to make that happen. your mother is not to blame for telling the truth of your past. you are the AH for wanting to and assuming you can hide this. get help, get therapy, and be honest.

you are not lost OP you are choosing to be. you love her so show her you love her by being proactive with your mental health and being honest with her. your mom does not wanna lie because she is a good person. you on the other hand are acting like a coward to your past and letting it dictate your future.