Also, have you considered getting housemates? It's terribly difficult for young people - and even many not-so-young people - to afford to live alone these days, and lots of people are looking to intentional community building / co-housing to make it work; I'm sure that's very true in your area as I've heard Seattle and the surrounding area is very expensive.
Obviously it means giving up some privacy in the common areas of your home, but it could be a help with income shortfall AND with forcing yourself to be a bit more social. Obviously be careful in vetting potential roomies (you can access public records for criminal and credit issues, and get & check references) and yes, there will be some aggravation - but that's true of people living in family units, too. I rented rooms in my house before having to leave Montana, and met a couple of nice people that way - it forced me outside my comfort zone which can be very good for introverts.
I'm not sure what to say about the other things you've mentioned; I'm not a terribly lonely person because I love to read and watch videos and generally enjoy my own company. I've been regretting a love I 'lost' a very long time ago and which I believe was my best chance for traditional marriage/family; but recently I realized if I'd taken that path I'd likely still be married to a Trumpanzee today, with Trumpanzee children and 26 years of football watching under my belt. I doubt I could ever have become a Trump fan myself, I would likely have been a long-suffering, silenced liberal like my mother was . . . anyway I know I don't regret the life of the mind that my education affords me, I only regret the shackles of student loan debt and that's just the sad result of the point in time in which I was born.
Honestly when it comes to relationships - I've had several since the 'one true love' and they were all nice guys who wanted to commit to me, but I balked at the starting gate every time - because even nice guys seem to want things more their way. I'm not saying this to have all the married/committed people feel the need to be defensive, but honestly, I've not seen but a couple of marriages I would envy on any level - seems like someone's always eating a lot of shit, or both people are. I'm not averse to companionship, but I'd just as soon have it with an abundance of freedom and independence and lots of folks are threatened by that. Funny thing is I'm super loyal when in love, but just need my space - lots of men can't deal with that, so I just fly solo and will do so until I bump into a nice guy who doesn't feel compelled to clip my wings.
My advice is to get out of your head as much as possible - oldr's right, it's a dangerous place to be for too long! Read, write, listen to music, dance alone - its great exercise! Meditate or take up yoga. Realize that we are ALL alone, and the most important relationship of your entire life is the one with yourself - LOVE YOURSELF!

One more thing: cats are ok, but the journey of life is best traveled with a dog, truly. Consider adopting one from the shelter, an older dog can be a wonderful low-key companion who makes you get out of bed to greet the morning every day, and will always be happy to go on your walks, for a row, or for a ride!
And finally, if you love kids and wish you'd had them, consider fostering one or more - even if just as a respite or short term placement.