How can I open up to people, if I know they'll probably reject me ?

It takes a lot. You just need to work on small things to boost you self confidence. Make projects with things that interest you and when finished you will feel accomplished and will work towards self confidence. When this is built you should eventually get over the fear of rejection. Wishing you the best of luck!

Open up because you'd like to share yourself. People who are happy social-butterflies open up easily because they like being expressive, not because they'd like to be accepted. And I've found out over and over again that rejection is not the worst. It may feel like it for a while but eventually it goes away, and you learn to be more confident each time, because when you know what rejection is like, you're less fearful of it. Of course that doesn't mean coming to people knowing 100% chance of rejection. That just means be a little more adventurous and take more calculus risks. You take better risks the more experience you have.