But how can I believe in the story?
Admittedly, that's the hard part. For me personally, depending on the change in the story, it could be fairly quick to a really long time. It wasn't right away you believed all the negative things, so it won't be right away you believe anything positive.
And if you're like me in this, in the beginning, there's also a voice that fights you for trying to believe anything good. I say, "I'm allowed to be happy," but instantly hear a rebuttal of "Ha!" or "No, you're not!" or something similar. Hopefully you don't have that, because it makes things take longer, but it is possible to change the story even with it.
I think, giving yourself room helps. What I mean is, it's ok if you don't say good things every day or are especially down some days, etc. I would make things worse by beating myself up even more for not being consistent and comparing myself to others and of course that made things worse. Besides being judged harshly by some that made my previous self-hatred even deeper. No one hated me more than me but it was others judgements of me that made me feel that way and I didn't realize it. I say these things so you know that I'm not just smiling and saying, go for it and not knowing how difficult it is to actually accomplish, but that even though it is difficult, I believe you can do it as well.
I agree with leaving the "but" off of your sentences, though it will still happen a lot at first because it's what you're used to. Only looking at the positive helps, because the negative is usually at the end and what your mind will ruminate on. I still struggle with some positive things, but I have come quite a long way. Instead of saying and truly believing that I hate myself, I can say that I like me and am learning to love myself. I don't mean to be bright and cheery for everything, just to start saying more positive than negative to start changing the way your brain thinks about you and your situations. =)
Also, though you say the same things you are unhappy about a lot, you are also saying what you have done and are trying to do to change it and even looking for suggestions and things here. I don't believe that's doing nothing. I've learned that every baby step we take matters. When you look back in a year after taking one little step at a time, you'll be amazed at all you were able to accomplish. Some people can leap, and some of us take one step at a time. Neither is better, it just is and it's totally fine. We're all still moving forward.