I agree with So Worried.
Having tried the online dating thing, I'm personally a believer in relationships occurring naturally, rather than through a website that is specifically for dating. I don't mean you HAVE to meet someone in person, but just that if the entire concept of a relationship is treated as secondary, I feel like things work out much smoother.
My number one piece of advice would be to focus on meeting people in real-life activities that are not focused on dating. Join some clubs and groups related to activities you enjoy (do you like to go for runs? Mountain biking? Do you play music? etc...), just make friends, and don't be too obsessed with finding a date. That being said, I know how SAD and GAD can be, and I understand if this is not the best option for you.
Alternatively, like So Worried said, find a social website that is focused on a certain hobby or just chatting in general. Start to get to know people through those sites by connecting over things you have in common. Try to treat the person as if they probably won't want to date you, like a friend. If things go well and you sense that a strong friendship is forming, you can both share contact info outside of the website (such as your phone numbers, *****, e-mail...etc). At this point, if there is any sort of chemistry between you two, things should happen pretty naturally.
Of course, that's just my opinion. Everyone handles dating differently.
I think the key thing is to not get too frustrated if things don't pan out. Trust me, I've been there and I know at least somewhat how you feel. Frustration creates a terrible snowball effect. The more frustrated you are, the more desperate you become. Desperation is the bane of anyone trying to get a date. I'm not calling you desperate by any means, but I know first hand how frustration can easily lead to desperation.
I totally understand the need and appeal of online dating, and I know it can have advantages for certain people. The one thing, however, that will always be wrong with it is the inability to sense real human emotion (I guess unless you're doing some sort of online video-chat dating). Things can get very easily misinterpreted as text. You might think you're being charming and nice, but the girls reading your messages may have this mental image of a very desperate guy saying whatever he can to get a girl to like him (not saying that's the case, but it could be!). There are a thousand different ways you can tell a girl she has pretty eyes in person. The tone of your voice, facial expressions, and body language can mean the difference between, "you have very pretty eyes, and I really mean that" and, "you have very pretty eyes...will that get me in bed with you?". If it's said through an online message, however, it's up to the girl to interpret it without all of those extra clues.
So if you're feeling down about not succeeding with online dating, don't be so hard on yourself OR the girls that aren't responding. Again, my suggestion (after real-life activities) is to replace the dating sites with "social" sites that don't treat dating as the ultimate goal.
I really hope this helps. Feel free to give your input.