Some of the lessons I've learned from therapy and self reflection:

Be careful about "pickiness" when it comes to making friends. When we are feeling alone , depressed, etc. it can be easy to fall into black and white thinking as a defense mechanism. In my case, quickly thinking of people as "potential friend" or "not worth my time" allowed me to protect myself from the rejection of someone not wanting to be my friend, or the more painful rejection of slowly drifting apart from someone you cared about.

Not every person that you include in your life has to be your best friend. When I was at my lowest I managed to start playing pick-up volleyball at the local rec center. Seeing the same people every week really helped me start to feel less isolated. Just having a familiar face to nod to at a gym, library, coffee shop, dog park, or whatever can really help.

Humans are social creatures. If you convince yourself that being alone suits you (another defense mechanism) it can be really difficult to recognize your innate need for human contact. Being comfortable when you're alone is good; becoming comfortable with loneliness is bad (if not impossible), so don't try to force it.