It's hard when we are constantly worring what other people might think about us, and in that way we can neglect our needs or sometimes people feel like they are not representing their true self to others.
Usually, atempts to "erase" the feeling which is difficult to us, like feeling of worry, leads to magnifying that feeling. So what is actually good thing to do is to acknowlage that we have that feeling (feeling is neither good nor bad, it's just here with us and it comes naturaly). Breething, relaxation techiques, meditation and mindfulness are very helpful with that. You can find mindfulness and breathing tecinques in this site (just click -path in the upper left corner and go to -mindfulness exercises). That can help negative feelings to stop growing or to reduce them.
For expressing your thoughts you can go step by step. For example, you can first try writing them, than reading them outloud by yourself, after that you can choose some people close to you, people who you can trust and practice expresing thoughts to them, slowly you can expend circle of people to who you are expressing your thoughts. This can be hard, but slowly it can reduce the worry about other what other people will think.
You can also try to notice what went well in those situations. How did you feel, did your worries decreased, what did you get from exspressing your opinion.
Also, working on personal selfesteem can help. For example you can take 10 to 30 minutes each day and write good things about yourself (if something bad comes to mind don't write it, just focus on good thigs) and things you are grateful on. Every day reread the list of things you've already wrote and add new things you noticed. For example I'm crative, a good friend, good english speaker, I can cook a lunch, I passed my exams, I've fixed a problem with my friend, I'm working on my assertivenes, I'm trying not to hurt other people feelings, I'm living near a beautiful river/sea/mountain...