Bless you. This isn't about stretch marks, this is about how you value yourself - or rather how you don't.
I'm quite a bit older than you (now 49 - yikes!) and definitely as I've gotten older, I've started to give less of a shit about being valued by some ridiculous beauty standards. I struggle with my varicose veins but maybe this summer is the year that I'll go out wearing shorts without caring what people think!
I've had lots of therapy (for other reasons) and there was something that I found that helped me. The therapist asked me to imagine my inner child, the little girl that I once was. Every time I thought something horrible about myself, the therapist suggested I imagine saying it to that little girl. I found that a REALLY powerful exercise. The suggestion was that you love and take care of that inner child within, which is essentially your self-worth and self-image. You are still that same little girl but just older. Could you judge that little girl so harshly? Could you say such awful things to her about being ugly?
Sorry, I know that probably sounds really wanky but that was one of the therapist's exercises that really hit home for me. I was in tears. Because I was like you and struggled to take care of myself, or see that I was worthy of love. And beautiful just as I was. Just like you are.
Also, don't ever feel as if you have to apologise for taking up space. As women we're conditioned by society to apologise for our existence and to be quiet, and not to talk too much about ourselves. Honestly, just fuck that. Therapy is the space for whatever you need. If that's listening, then fine. But don't feel as if you "mustn't" speak. Your voice is worth hearing ❤️
As a PP said, CBT can be helpful as it's less about dwelling on your past and more about actively tackling negative thinking. Another idea might be hypnotherapy. I've used it for other issues and found it incredibly effective. You need a practitioner that you feel you can trust - which might be male or female, depending on what you prefer. But when you find someone who you feel at ease with, hynotherapy can be fantastic for tackling deep-seated beliefs and doesn't require you to do any talking.
The reason that I think tackling your self-image and esteem is important is that even if you magically get rid of these stretch marks, then what next? There will always be something that makes you feel "less" until you're able to love yourself and show yourself the same kindness that you clearly show to others. .