I'm a shy person too, so I can relate. Basically, a smile does wonders. Practice smiling at people. If you're out dancing and see someone you're interested in, smiling at them is a very simple way to make them comfortable enough to approach you. If you're close enough to the guy, saying something simple like, "I really like your shirt." is a good way to start a conversation with someone. Everyone likes a compliment, and it's not a creepy come-on line. He won't know if you're interested, or just a friendly person, but if he's interested, he'll definitely like to continue the conversation with you.
Everyone fears rejection. You can't live your life in fear of that. If you don't take risks, you will be where you are now--alone. Try not to take it personally. You are attractive, but you can't be every man's type. Just like you find some men attractive, but don't have chemistry with them. It's biology.
You know what's funny? I tend to be nervous around guys I am interested in, and don't smile as much. I am able to talk better with those I am not interested in, which I think leads to some of my problem, because they end up liking me.
I was you in my 20's too (now I'm in my 30's# and I'll share my advice with you as someone who has been there and gone through exactly what you are going through right now.
men are not complicated to figure out.
Everything takes practice. Smile at elderly people in the grocery store. Just get used to smiling more, so it comes more naturally. Think about a confident woman, maybe a friend of yours or an actress that's relaxed and friendly. When you meet someone cute, try to channel that confident woman. Temporarily pretend that you are her.
I had the same problem of not smiling enough. I'm in my forties and got divorced 4 years ago. I experimented one night with smiling while at a club. I was waiting at an upstairs railing for my friends. I saw a younger guy at the railing across the way. He was too young for me, but my intent was just to see if he would smile back.
He smiled back. A few moments later he caught my eye and smiled at me, and I smiled back. My friends arrived and we went downstairs to get something to eat. I didn't want him to approach me, as he was too young, but it seemed to make him feel good that I was smiling at him. It was fun to try out. I have a boyfriend now, so I don't have to worry about that anymore. I hope you find a good one!