The reason for you to having a reversed feeling toward this, is because you’re stressing too much about things. Whether you were stressing about what your partner think of you, or you’re stressing you aren’t doing well, or you’re stressing thinking about the future and what would happen with this partner, so you condone the good side of it, and only focus on the bad side. This is the reason why you’re not happy with other people, including friendships.
You feel better on your own; because you don’t feel the responsibility of other things but yourself. So there is nothing to go wrong. Even if something went wrong, it’d be with you, and you wouldn’t have to worry much about it.
This is actually concerned with anxiety. Doesn’t have to be social anxiety; because you might be in good terms with others, but on the other hand, you aren’t feeling the joy.
It takes a while to sort this out, but it’s fixable. No need for medications and stuffs. You just need to put in effort. What would you lose anyway?
So the first step to do, is try to worry less about these things, specially these things you can’t control. If you’re talking to a friend, and he isn’t feeling good, but you have been trying. Don’t feel bad; because you couldn’t make him feel better. Ik it’s kinda insane and you’d end up feeling stressed and something is wrong anyway. But you gotta know that nothing works as planned, all the time. Things will go wrong at some point. But the thing you can focus on, is how to sort it out in the best way with the least impact.
About being in relationship, it’s actually alright; because nothing is totally perfect and sure things won’t always be horizontally running. Being in a relationship could be put on a graph of a zigzagged line. You just try to minimise the deviation. So when things go wrong. Don’t be worried that much. It’s normal that everything might go wrong.
It’s only matter of time and trial&errors. People come and go. So don’t stress that much. Worry about your health.
GL