I think a lot of your unhappiness comes from comparing yourself to other people. You may think that other people your age have had so many more experiences than you and are doing much better than you are, but they're probably looking at your life and thinking the same thing. When you're looking at someone else's life from the outside in, of course you think they have it better, you're only seeing their highlight reel.

I think you also need to reframe/figure out what happiness means to you and take action to achieve it. From your comments, it sounds like you just think of things that might make you happy, and then wallow about how your life is missing those things. If you think something might make you happy, then go out and pursue it!

If you want to make new friends, what about your girlfriend's friends? You could try hanging out with them/getting to know them better. What about old friends that you've fallen out of touch with? You could try reaching out to them. If those don't work for you, you can also try picking up a new hobby that involves other people, joining a club, volunteering for a nonprofit, etc.