Honestly I fee like these are normal feelings that everyone goes through at one time or another in life. Not that this makes it any easier for you at this time in life, but realizing that others go through the same hardships can at least make you feel not alone. I think that the subject of not being happy in life if one doesn't get married is a deep seeded fear in most if not all of human beings, especially as we get older. We place so much of our percieved happiness outside ourselves onto another person, real or a hopeful person, that even if we end up with that person or married in general, we usually discover that this person may bring us happiness but not all that we wanted.
Or we can come to the realization that being married didn't bring you the happiness you thought it would because of many reasons, the main one is that usually that you yourself aren't happy or don't love yourself enough yet and are expecting a person to fill that void. Im not saying that this is the case with you, but this is one of the biggest reasons the divorce rates are so high. We want the other person to fill that void and bring that happiness to us in which we seek, and if it doesn't happen we end up resenting that person.
Turning in and becoming more happy and content with yourself first will not only allow you to become a more a happy person but you will also attract better and happier people into your life, potentially a great mate.
Like Tony Robbins says, "In order for things to change, you have to change"
Hope this helps my friend.