Hello!
Your question are those, the answers to which most people are seeking. Lets take it one by one.
I don't know much about you, so can't pin point here the exact reason behind your social anxiety, but generally speaking many people have inhibitions being social. Always remember that everyone has a unique personality and so do you. The thoughts you think are stupid, may turn out be very bright ideas in someone's eyes, so speak your heart out. Always be yourself. Whenever you are at a social gathering, show your presence with warmth and a positive attitude. Be who you are and don't pretend to be someone else to impress others. When you will be at a gathering there will be many who will be in the same boat, so don't worry you are not alone. Dress well and put on a bright smile on your face and speak with confidence. You will win any situation.
How sure are you that whatever you say is stupid? You have asked quite intelligent questions here. Share your thoughts and give others a chance to show you that your ideas are worthy. Don't ever underestimate yourself. Tell yourself that you are an intelligent person who is capable of making the right decision.
How to live happy? Happiness is a different definition for everyone. Anything and everything can make you happy, so take your journey inward with that I mean, make yourself happy. No one in this world can do that for you unless you want to do it for yourself. Let others peoples opinions and social media stories not affect you. Set goals for yourself, live for yourself, listen to your heart, follow your dreams. Learn to accept things that you can't change. Love, laugh, live in the present moment and don't regret the past or worry about the future. God bless you! Stay happy!
Next Steps
What you are feeling can be very easily sorted by talk therapy. Connect with a therapist and you will get workable solutions to all your questions.