I feel some of your pain...I, too, have mostly introvert hobbies and have trouble making new connections. Am currently single, after ending a poly relationship that was hurting my self-esteem, and don't expect to meet anyone new for a while.
My only advice is, keep trying new hobbies, even if they are solo hobbies. Mine right now is cooking, which for me is a solo hobby because a) I get stressed out cooking for friends, b) my place is too small for good hostessing of parties, and c) many of my friends have a lot of dietary restrictions that make cooking for them extra stressful. But it keeps me busy and mentally and emotionally engaged, so I don't feel lonely or bored. And the feeling of mastery I get when something works out gives me a big emotional boost. I've also been working out more, trying new things and new workout locations.
I also probably have what psychologists would call dysthemia, or low-level, long-lasting depression. It disappears when I'm in NRE (which can come not only from a new romantic relationship, but also an exciting new job or new friend that I am excited about spending time with). It sucks to feel like my mood is out of my control in that way, but it mostly seems to be.
That said, there are a lot of things you can do for depression that don't involve seeing a doctor. Getting aerobic exercise, if you don't already. Getting out into nature when the weather warms up. Keeping a gratitude journal on a daily basis. Doing written exercises to address the tendency to put yourself down. (David Burns has several great books you can use.) I personally have found most therapy to be at best slightly helpful, but these techniques do help somewhat when I manage to do them consistently. SSRIs have been a mixed bag for me, but they're worth a try if you haven't tried them.
You don't post your age but I gather that you, like me, are past the age where there are lots of single people floating around looking for a relationship. Even poly people can be "polysaturated" and not looking. So it might be a while before you meet someone. But I believe that the more you do to try to figure out what makes you happy that is within your control, the better prepared you will be to be happy with someone else, when that person comes along.