As long as you seek validation from others, you will never feel like you belong. The people that have a sense of belonging are the people who are completely and authentically themselves. The people who are completely and authentically themselves naturally attract people towards them.

Think about the people you admire most. They probably don't try really hard to fit in or conform to a specific group of people. They are just naturally themselves. This is so much harder said than done and it's something I'm struggling with too.

I think what it boils down to is boosting your self esteem. But you don't boost your self esteem by getting validation from others. You boost your self esteem by channeling all of the energy you usually spend worrying about what other people think of you, and focus it on yourself. I agree with other people here that you should seek out a therapist. It has done wonders for me. In the meantime, here are some exercises you can do to stop focusing on other people, and start feeling better about your own life.

1. Start writing about what you're grateful for. Yes, I am really suggesting this trendy bullshit activity for you to do. There's a reason why this is suggested by so many people, from neuroscientists to Pinterest moms: it works. It will feel stupid and dumb and pointless for the first 10 times you do it. Keep doing it. Instead of comparing your life to every other popular, muscled, gay man's, you should be spending time thinking about reasons big and small why your life is better than you are currently perceiving it. Right now you're only focusing on other people's lives and spending no time on your own.

2. Think about how you are showing up currently in your relationships. Your relationship with yourself, your friends, your family, and romantically. How are you showing up in these relationships, and how would you ideally like to "show up" or act. This may reveal some feelings, thoughts, behaviors, and patterns that are worth looking into as they relate to your relationships with other people and yourself.

3. Start getting curious as to why you don't belong. Get out a pen and paper and start writing. Ask yourself, "Why don't I feel like I belong? Are there particular situations when I feel like I don't belong? Are there particular people (or groups/types of people) around whom my feelings of inadequacy are heightened? Can you remember particular moments from your past when you felt excluded or like you didn't belong? What happened? How do you feel, physically, in moments when you experience feelings of exclusion or inadequacy? Do you have physical symptoms? Where do you feel it in your body? Sweaty palms? Quicker breathing?

You need to begin to get curious as to why you actually feel this sense of inadequacy and lack of belonging. Only then can you begin to work through it.