I was really shy - it should be on an official disability list somewhere, I swear. It can make your life hell. I'm still what you'd call a reserved person, but it's not debilitating anymore.
I found my biggest issues were around not knowing what to say, and being afraid of people judging me inaccurately. The wake-up call came when I realized they were judging my shyness instead - assuming I was a b1tch before giving me half a chance.
I hate small talk. I suck at it. I never bothered to say anything out loud that I didn't think was important or worth discussing. So I had toast for breakfast. Who cares. But when you listen to other people who seem to be great conversationalists talk, they're really talking about nothing. They're enthusiastic about it, but the subject matter could be anything. It's usually a story about something mundane they did that day, and people hang on their every word. I still don't get it, but there it is. Maybe it's the enthusiasm.
A couple things I've found... most people love to talk about themselves. So one of the best ways to hold a conversation is to ask questions about the other person - then questions about things that come up in the conversation. Let them have the spotlight - they'll love you for it. You can use it in a way that's beneficial... you're not just another person waiting for their turn to talk. You're deep, and genuinely interested in them.
Or if you know you'll be struggling, check out the news before heading out and keep a few gems in reserve. They're good conversation fuel - and they're not personal so it's pretty safe.
If there are self esteem issues involved those are obviously going to hinder your confidence level. So project yourself the way you'd like to be perceived... eye contact, good posture, easy smiles. And remember you're just as good and worthwhile as they are. It's all really elementary stuff and I feel stupid writing it, but it does work.
It's like anything - the more you do it, the more comfortable you become and the easier it gets.
It seems to me you have a fear of putting your feelings on the line when you talk to girls. Practice does make perfect. My advice is to talk or meet girls as much as possible, in a friendly way. Find some common ground to keep the convo continued (new movie, music, travel, etc. ). And if there is a girl you might like, man up and go for it. It's all about confidence. Even though a beautiful girl looks intimidating, once you get to know her it won't seem that way anymore.
I have an answer for you that will take a little time but make your life infinitely better. Message me if your interested. Don't bother if you don't know you don't have the time necessary to get better with girls. It takes time like everything else. I think I can help you though.