Are you physically active or interested in being physically active? Hiking, climbing, fighting, dancing, running, swimming, weight-lifting, soccer, kickball, ultimate, softball, cycling, bowling, tennis, etc.
Are you dating someone, occasionally going on dates, not dating at all? The first two can certainly occupy your time, and can help you find activities you enjoy by doing things with your dates or partner. The third, if you prefer things that way at the moment, keep it that way, otherwise maybe spend time out socially trying to meet partners (church, bar, club, sports league, etc.).
Solo indoors: reading, video games, TV, education (organized like an online masters program, self-guided by choosing a topic and reading/researching it).
Solo outdoors: Physical stuffs above, gardening.
Creative hobby: cooking (can be creative, or just enjoyable), painting, drawing, sculpting, wood working, metal working.
Hell, take up a technical skill like welding or something. There are places near me where I can learn that in the evenings, probably available in most decent sized towns. Auto-shop classes if you don't have that skillset yet and may want it.
Some of these things you can just try, give them a month and see if they stick.
Spend more time with friends or family being social. Host movie nights or game nights or barbeques. Organize the night out events.
Sometimes it looks pretty similar to my day job, but to the benefit of a different organization, like when I mentored a FIRST robotics team.
Recently it's looked very different. I balance work with sports so that I can keep my body working well enough to enjoy a longer career :)
I did, however, just leave a job where I was unhappy. The timing was not great and it cost me. But I couldn't be happier to be free of that job. When I started to seriously dread going in to work, I knew it was time to move on.
Try to realize that not leaving a job because you "can't stand disappointing anyone" is just plain wrong. In a sense, you're doing them a disservice by remaining in a position you are not happy in. You are probably not performing at your best if you're unhappy. Plus, startup or not, they would probably drop you like a bad habit if given the financial incentive. Business is business, money is always the bottom line in that environment. You do not owe them anything.
1) The money situation is tough, but workable. Are you in a position to ask for a raise or are there things you can cut back on to save some money? If you don't want to change jobs yet, try those.
2) You don't want to disappoint anyone, I understand that completely. Was this something you were once passionate about but lost interest in? Your post sounds like you're experiencing burnout. Do you get paid vacation? If there's no formal leave agreement, maybe ask for 1-2 weeks of paid time off to just go get your head clear. Drive up to the mountains and hike or relax in a cabin with some books. Come back a bit refreshed.
3) What kind of hours do you work now? If your work is taking up your weekends, reclaim them. Take day trips somewhere, find a local rec sports league, or go to a park and find a pick up game of soccer or something. You may not enjoy these things, find something else. But try things. You won't know what you like until you do. I had no idea I'd like ballroom dancing until I took a date to a social dance one night (because I knew she liked dancing). It was the first time I'd danced in years, and it was a blast (for me, also terrifying because I felt like a klutz).
4) I get the bit about no serious romantic relationships. In all honesty, I had 2 in high school and college, nothing post college until that ex in the last paragraph. It's not easy, and it can be terrifying returning to it after a long hiatus. But it's one of those things that you have to try for if you want it, relationships (rarely) fall into your lap. In fact, the first time you have a partner and not just someone you go on a couple dates with may end in disaster. But I don't regret that one, parts of it, but not the relationship itself. I was trapped, inertia-wise, in singlehood and dating her got me past that hump.
Well, there's three things for you do start doing beside work!
There's a big different between having something you can riff on for a while and then drop when it doesn't hold your interest and something you have to ship regardless of what's involved.