Just remember my friend, when one door closes another opens. It's a lot harder for a woman your age to find a good man (although fairly easy to find garbage) than it is for a man to find a good woman. Get away from your computer for awhile today, go someplace, just mention to someone that it's your birthday and that you just broke up with your girl. Something tells be you won't be alone for long. Relationships with women are much better in theory than in practice. After 35 years with the same (very good) woman, I'd just get hookers if anything happened. Who need all the baggage they bring to a relationship. Plus it's cheaper and nobody complains if you just roll over and go to sleep (snoring) afterwards.
At 51, party hearty. Life is a bowl of cherries. Celebrate!