I'm thinking of declining close friend's wedding invitation in fear of facing her partner
Why are you afraid of seeing her partner?
I don't know how to face him, I feel too shy in front of him.
You should be there for her big moment in life. Just tell them "Congratulations" if you don't know what to say.
Your right. I'm an adult, but I'm acting like a child.
I'm glad to hear you say that. If you have extreme social anxiety, you should check with your doctor about possible remedies.