Anyone who is already lonely or lacking self-esteem should stay well away.
I was in a very similar situation to the OP, an introvert who's career and interest led to a state of near constant solitude. I lived this way for over 10 years. I had a weekly routine; weekend was going to the store and househould maintenance, weekdays were going to work and fiddling with side projects after work. I would visit family every few months. But otherwise I was entirely alone. I feel that I am a true introvert, and relished it for few years, but it ground on me. Later I realized that I could die and no one would know for weeks. This was profoundly saddening to me.
I used online dating off and on for years with no success, and a lot of frustration as the parent indicates. It can be crushing to message 20 women and get zero responses. Over the years I worked on tweaking a message (a/b testing essentially) to see what would get responses, got some flattering photos to use. Then after I started getting some responses, and going on a few dates, still nothing. At one point I gave up for good, decided I was asexual and unlovable, and resolved to live out my life alone.
I won't say I was ever depressed, I do have a melancholy disposition. I experienced one episode of real depression for maybe a day, it was a dark pit, I couldn't stand to live. Luckily it only lasted a day, and never returned. That episode showed me how bad depression can really be. Not a constant state of being down or blue, but grinding darkness that makes your own skin unbearable. It was terrifying.
What drove me out of all this was series of illnesses with my parents. I had always imagined I'd get married have kids, but had zero success dating. I realized if I ever had kids, they wouldn't know my parents if I didn't act. So I got back on the horse, systematically used plentyoffish to date a lot (edit; well, less than 10 so not a lot) of girls and eventually met my wife and we have two kids.
Nowadays, I'm happier than I've ever been, but things aren't perfect. Being married is hard, particularly since my wife and I were both older when we met and "set in our ways". She's not the easiest person to live and neither am I. Our kids are a joy though. I get zero alone time now, which also grates on me. I REALLY need it, and my spouse is not sympathetic.
My wife says about herself "I was lonely wierdo, until I met you, now I'm not lonely, just wierd", which also applies to me, which amuses us.
Anyway all that's my .02, you're not alone, and that was my path.
I tried online dating about 7 years ago and found it ok, had a few interesting dates but nothing special. I then met my wife (offline) who I thought was my soulmate.
After a few years she left me, and I decided to try online dating again. It feels very different. The women have this incredible sense of entitlement. I don't really blame them, apparently they receive so many messages that it's impossible to reply to them all. It's common to see something like: "If you're just going to say 'hi' don't bother messaging me.", but you can also waste a lot of time writing thoughtful opening messages that just get ignored. It's a painful daily reminder of exactly where you fit in the pecking order, which is why I wouldn't recommend it to someone who is already lonely.
It's hard to pin down exactly what has changed in between the 2 times I tried online dating. But I don't remember it being such a challenge just to get someone to reply to me.
In regards to dating sites, I've tried using them on/off for probably a year or two now. I've had a couple dates but they fizzle out quickly. Unfortunately, my age-group is big on the "hook-up" mentality which really doesn't appeal to me. Maybe, as you said, I need to just get back at it.
