I feel for you jrccomputer. Reading your posts toke me back when I struggled with those same issues. Being shy at a young age, it took me a while to mature socially when it came to dating. I had the same insecurities and anxieties you face today. What helped me get through it all was time. Over time, God put women in my life that helped me grow and gain confidence. Even though each relationship lasted varying lengths of time, each one had value.
My first bit of advice is to relax and try to have fun. I don't know if this was supposed to help or not but I read a Christian piece on how very few relationships end up in marriage. I think that it was an attempt to take the pressure off, to keep us from worrying that if we make one mistake, we will never find another potential mate, and will be forced to wander the earth alone.
Every time I ended a relationship, I worried that I wouldn't meet someone as good as they were. The reality was though that each one that followed was more like the woman I wanted to marry. It could be that I needed to experience a relationship with someone that had certain traits that I thought I was looking for, and God allowed them into my life to correct me. Sometimes, I just wasn't ready for them. Either I needed to mature, she needed to mature, or other things needed to occur that would allow a strong union.
I'm sure you've heard how important friendship is to a marriage. With that in mind, approach each woman you have an interest in as a potential friend first. Take the pressure off of yourself. Chances are, they are just as insecure and nervous about this process as you are. Just be yourself, unless you think that you can keep up a facade your entire life.
Don't you want people in your life that accept you for who you exactly are?
Finally, you never know where God will bring your wife from. She may end up being a friend of someone you meet on a dating site. She could be a waitress at a restaurant where you take your dates, and she becomes very impressed on how well you treat them. Consider all of this as practice. You'll keep getting stronger in your confidence. You'll find better ways to express yourself without getting nervous. You won't come on to strong because you'll realize that if she was meant to be the one that God has planned for you, you'll come together when the time is right.
My heart goes out to you. I still struggle with relationships as we all do, and it only ends when we no longer reside on this planet.
Good luck and God bless.