Feeling like a burden in my relationship

I've been feeling very depressed lately, I have a partner in better terms since its not official, but also not hidden. They know I deal with many issues, in better words I'm pretty much of a burden in a way. Or so I feel. One issue with me is I overthink a lot, and many people say I should communicate and ask my partner for reassurance, but i feel as a burden already as it is, and adding that makes me feel like they will get tired of me. Every time they talk about their relationships with their friends i cant help but feel like one day they will leave me for someone better and mentally stable. I really do love them, and i really am trying to get better. But i cant help but feel like such a burden to them and inconvenience.