I hope that my input isn't unwanted, but you seem to put a lot of these vents up involving you devaluing yourself, and worrying about your relationship. I just want to tell you that the worst thing I ever did in my one relationship was devalue myself. They say you have to love yourself before loving others, and that is true. You can't depend on others to be your emotional crutch, and you can't just dislike yourself whilst in a relationship. All that does is strain the relationship until something gives.

This is advice from someone who had similar troubles, friend. If you ever wanna talk, I might be a stranger, but I'm open to talk whenever. You're a lovely artist, by the way. You do well at getting the mood of your venting out through colour alone, and the texture really makes it feel complete.

I can't really help it. I have the problem of overthinking and when I do... I think about all the little things that have been said or done.
Sometimes, I feel like it would be better if I wasn't even there. And then other times, I'm really glad to be there.

I always put others before myself, since I'm happy when others are happy. But now I know that I'm happy when I know that I'm the one that has made them happy. I don't know if that means that... I'm like, selfish? Or something. I can't really word it right.
I enjoy seeing them happy, but if I'm not making them happy, then I just feel... I don't know.

I understand that I can't make them happy 100% of the time.

I know a change of attitude and outlook is easier said than done. I think that devaluing yourself would have the opposite effect of making people happy, so that's something to consider. I don't think it's selfish to want to be the one to make people happy. I think you're saying that if someone is happy, but because of someone else, that makes you sad? I think that's understandable, for certain, but just because you weren't the cause of someone's happiness doesn't mean you're better off gone. On the contrary, I believe you have people that care deeply about you sticking around with them for a long time coming.

When I was going through similar issues, I didn't really consider any advice until much after it was too late for my relationship. Trust me, putting yourself down is one of the worst things you can do.