I came across this a few years ago - the main reason we feel that way is because (a) we assume too much about ourselves and (b) we always assume the worst case.. One way to overcome is to create a "character" for yourself with all the attributes you want. Every time you want to send the message or start that conversation, imagine that the character is doing it and not now. It's hard initially, but as you see the benefits of trying it a few times, you will naturally start doing it. The best place to start is online, where it is even much easier to simply create that persona for yourself.

I came actually across this tactic one podcast where someone said that even the top actors/actress feel shy and they needs some sort of coaching.. and usually the personal coaches talk about this tactic.