I'm sorry that you're single and unhappy about it... I've been single going on 6 years and it gets to me every now and then (especially around the holidays!) But it's my belief, too, that there is someone for everyone.
A couple of questions to ask are:
1) Am I doing anything about it? Am I doing things that will get me to meet people? Joining clubs, going to churches, community events, and so on?
2)Am I being the type of person to others that I myself would want to marry? There is a quote (I don't remember the exact context, but it's similar), "I went out to gain a friend and found none. I went out to be a friend, and found many." Same thing applies to love in my opinion.
I hope this came across as friendly advice and not anything preachy because that is certainly not what was intended!
Missy