But then I got a girlfriend, and soon after she moved in with me. She lived with me for an entire year. Every day I'd wake up with the same person and go to sleep with the same person. Anytime I wanted to go out and do something where you wouldn't want to do it alone, like people watching at parks, the beach, movies, new restaurants, road trips, new bars, etc., she was there and we would do it. I took up some new hobbies. We vibed really well for the most part, so lots of good and spontaneous interactions on a daily basis.
Then a couple months ago we broke up and she moved far away. Now I actually experience loneliness. I can't just go back to the old way, and isolate myself for a week at a time for some project or to learn a new technology. Viscerally, my body can't put up with it. I used to sleep incredibly when she was there. I could easily crawl into bed way before midnight or take naps during the day. Now I'm up late reading stuff or working on something every night, and I don't feel that great about it, physically. I have lost the desire to continue any of the hobbies. I feel a constant subtle pressure like something is wrong and I need to do something to fix it. It's not because I'm butt-hurt about the breakup (it was welcome at the time), it's just not having a person like that in your life anymore.
I think the only reason I was able to go on the way I did prior to the cohabitation, was because I'd simply never experienced such raw, intimate human connection before, like the kind that comes with living in the same one bedroom apartment with an SO for a year. It felt like it awakened a truly human need that perhaps had been long suppressed or I had rationalized a way to keep a distance from.
At any rate, the experience will hopefully make it easier for me to find another gal out there, now that I recognize, or rather "feel", the importance of deeper human connections.
I do think you will acclimatize to being alone over time, and it gets easier and perhaps almost back to 'before'. But I also agree that once you've had a taste of good partnership, it changes your perspective permanently, and you don't want to settle for 'before'.