Feeling beaten down and inadequate and need of sound advice.

Hi everyone.
I need advice or something kind i can remind myself of everyday that can help improve how i manage my emotions and have clarity. Im struggling to deal with the affects of past trauma, a stepdaughter(16yrs) who has projected her feelings onto me that has done and said things that have caused me to feel insecure about the relationship with my partner, a partner who isn't partnering very well with me and very low self esteem. How do i overcome feeling undervalued unworthy and being made out to be a villain.
Past trauma was a 6yr relationship with the father of my 2 kids who i can only compare to a narcisist. He's no longer part of our lives but still hoovers and gaslights.
At present my relationship is in it's 5th yr with a good man but im afraid he thinks his daughter is the poster child for pure innocence and cannot accept that his daughter could or would ever be manipulative or passive agressive to anyone let alone me. He believes that only of me and that me getting frustrated or angry is the reason.
I now have little energy to pull myself out of this depressing feeling and although I've put a lot into building a relationship with her has left me almost depleted im expected to let it go and let down my guard and continue to persue a relationship with her.
Is it wrong of me at this point to tell him that i don't think being a parent to her is not in anyones best interest right now because she triggers me and i need to find ways to cope and because of past trauma that was only discovered last year i really can't put myself in a position where i need to be pretty whole to do a good job. I can't be expected to work hard and put effort in if theres no cooperation from her and without judgement.
I haven't talked about love because we all have a lot of that in us to give and share. This has to do with negative perception, entitled behaviour and destructive intentions. I feel neglected used and bullied. Hormonal teen? sure but even an hormonal teen gives up and moves on to the next silly reason/behaviour/attitude whatever is going on and you can see the changes. This has always felt more of a personal thing. Personality clash? I've thought of that but on the occasions we did get along it was easy until i questioned her intentions. But hormones and personality clash cant account for daily sabotage and implying to teavhers and other family members that im abusive and she's scared (this just in the last month or so. Ive never laid a hair on her. I've become short tempered a shouty mumand i hate it. But everything about me she's been critical about. From my personality to my bedroom habbits.
I want to find a way to help myself through her negativity and be positive through it all.