I don't want to write too much and make it too confusing to understand what I'm trying to say, so let me share a little story in the hopes you can use something out of it:

I always thought that my shyness was caused by low self-esteem alone, but years ago (in the mid 90's), in one situation when I was basically forced by my boss to give a presentation in front of a 100+ people crowd, I discovered that it was also my lack of self-confidence, because of my lack of knowledge about the product I was made to present. I panicked big time, had to be replaced by someone else and soon enough, after a couple of days I was dismissed from that company.

Years later, I took an extra job at nights as a computer hardware instructor, and was in charge of a class with 15 people. I felt really shy at the very first night, but I knew what I was there for. I had the perfect opportunity, I felt empowered by the knowledge I had to share, and people were there to learn from me. I had no reason to doubt myself. So after the first few minutes talking with a trembling voice and shaking like a leaf in a storm, I noticed that people were interested in what I was saying, then my shyness disappeared.

Moral of the story above: if it happens that you know what to say, and you know what you're talking about, and also it happens that people are interested to hear you, shyness itself disappears. This is true not only for work life, but for personal life too.

Beating shyness, low self-esteem and lack of self-confidence doesn't depend only on yourself and it doesn't happen all at once, just like that.

It takes time, opportunity, and it also takes not giving up and embracing shyness just because it's the easiest way out of suffering.

My advice for you would be to keep on trying, and take your failures as lessons rather than reasons to give up.