My story is similar to Kaytee's.
I used to be so shy I could not hold a conversation. I far preferred to be invisible. I did not like people noticing me.
When I went to uni I chose a different course and uni to all my other friends. Not to be different, but that was just the way it went. So I had to make friends and stretch my zone a bit.
Then I got posted 6hours away from home, and heck, I was a nurse, I kinda had to speak to people, and strangers at that. I don't know how but it just kinda happened. I was doing something I loved to do so it wasn't so noticeable.
Then I joined the cops and well duh, have to be able to speak and be social for that job. One thing that changed me forever though was the self defence and forcing myself to look forward rather than at the ground (cops do this for safety but it's amazing how many people will smile back if you shoot them a smile). The boost in the "I can" (because I had to but that's by the by), really made a difference to me.
I went on to train a large group of often unknown cops.....

Anything that will boost your "I can" feelings will help. I found toastmasters a help when I was a kid too. A course in something you love, or even something you think is interesting to boost your belief in you. Something that makes you feel good (you know that brand new haircut invincibility?).
Then take that and smile at 5 people...5 strangers.
Practice that a little while, days/weeks and then move on to say hello to 5 people. Random people. You'll be surprised at how many will chat back.
I don't think there is an easy/comfortable way to do it. But a conscious "I want to beat this, I can do it" attitude will help stretch your boudaries.