A question I have asked myself is why do I let my self worth depend on my partner?

Lack of self confidence could be a factor. Its good to have confidence in yourself and not rely on anyone else on this, because people can always upset u at any time.

We all need validation. We are social beings; from infancy we learn that how others perceive us will help us grow into the kind of person we want to be. As we grow up, the way we let others influence us changes, but we still need confirmation that we're doing friendship and community in a way that others can relate to. We want to trust others' evaluations of us, so it can be hard to notice when someone isn't using negative comments to be helpful (like, "you can't wear that to a job interview!"). And someone actively lies while betraying that trust is even harder to recognize. It's not a weakness to believe someone who's been caring and intimate with you. It's human. And that's way getting away from toxic individuals can be so difficult.

I find the cause of this is a lack of self-confidence. You don't believe in yourself, so you try to do the next "best thing". You seek validation from other people. You want other people to say, "You're good enough" because it makes you feel good. The problem with this is your confidence and value become reliant on other people. You need to work on yourself image. Tell yourself "I am good enough" even if you don't believe it.

You need to learn to love yourself before you can love anyone else because if you don't love yourself no one can love you.