Late-30s, single, perpetually so.
I have had relationships but never for more than 3 years, give or take, and haven't lived with anyone (bar Covid) since 2009. I think that you're right OP, sometimes it's just really bloody tough.
I like my life, I have a good job, I earn well, I live where I want to, I moved (again) because I wanted to, etc. From the outside, I know that my life looks great and people say things like "I admire your independence", and "you're so strong", and "I wish I had your freedom/confidence". However it certainly doesn't feel like a glamourous, happy, fulfilled lifestyle when I get in from a nightmare commute, the lights are off, the fridge is near-empty, and no one offers a cup of tea or asks about my day or says "fuck it, shall we get a take away?".
I don't long for a relationship in a sappy Prince Charming way, but I do miss companionship, intimacy, complicity. It'd be nice to have some well meaning idiot cooking his one fail-safe dinner then fixing the light above the kitchen cupboards. Of course I can do those things myself, but it'd be nice if it wasn't ALL my problem.
I have made some good friends (recent move meant that I needed to start over) and luckily some of them are also my age and single, so they appreciate the need for connection. Even if it's just to hang out or grab a quick coffee when your paths cross while doing something else, it can make all the difference to my day. I had a GF stay this weekend and it was lovely to just not be alone most of the time.
I think the most important thing when alone a lot is to put yourself first. Even if I don't feel like doing x alone, I still do things because I want to. Cinema, hike, dance class, creative workshop, volunteering... all of these have enriched my life and created some social connections. It also means that days alone are not "wasted" because at least I did something that I wanted to do.
I also "force" people to celebrate my milestones and acheivements. Everyone "recognises" engagements, marriages, babies, anniversaries, etc., but when you're alone or not "ticking these boxes", things go unmarked. Throw a housewarming party, insist on gifts, tell everyone when you get a new job/promotion and that you expect them to come out for drinks to celebrate. It pisses me off sometimes to remind people I'm alive, but it's better than sitting indoors feeling sad about it.
Sorry this turned into an essay/rant. In short, being single sometimes sucks when you feel lonelt, but it is not terrible all the time. You have to do the things that make your own life fulfilling and fun for YOU.