Noranne, it is such a long game, and knowing it intellectually doesn't really help, does it? Especially when this is something you've wanted and worked for for so many years.
I think I mentioned this in a different thread, but about this time last year I was at the point of giving up on my book, which was the 7th one I'd finished but the only one I'd queried. It got an agent quickly, but she asked for many, many rewrites, and I was pretty convinced I'd never "get" what she was trying to tell me, and the whole thing would fall apart. I started a kind of journal where I wrote down what matters to me in life, what I truly wanted to achieve in writing, what was making me so frustrated. I asked myself what I would change about my life if I decided it's okay to never ever be published. Turns out I wouldn't change anything. I'd keep writing, keep trying. Realizing this eased some of the pressure I was putting on myself. All of that pressure is internal, and we can change it, even if it's very hard! Once I relaxed a bit, my writing improved, and everything worked out. But it was a long, long road.
Keep at it! Maybe some soul searching will help you like it helped me. Or take a break. Appreciate the other parts of your life that don't have anything to do with writing. Pick up a non-writing hobby. Sometimes great things happen when we don't strive quite so hard. We have to pace ourselves for this marathon, right?