I spent the day telling my housemate that I was my own Valentine's this year. It took me a while to understand why it sounded so sad and creepy because to me, it makes perfect sense. Love yourself and the rest will follow might be beautifully cliché, but I still maintain it's about the best advice one can get, and especially when single on Valentine's day.
Love yourself, or at least accept yourself. The loving part will take more time, but I believe it'll come around eventually. And give yourself a break. Today, treat yo' self! If you have no Valentine's, there's a large chance you are the most important person in your life. So take care of that person.
There's no shame to be had in being alone. I'm actually one for quite a lot of me time (which doesn't mean I don't have friends or don't like partying... Believe me, I do.) But I frequently need to be alone, go at my own rhythm, do things my way and not think about what anyone else thinks of me because there isn't anyone else there.
Some more important and wise advice on that matter:
If you're alone on Valentine's day and feel like crap, cheer up. Make it about happiness. Make it about food. Make it about friendship. Today, I got given a heart-shaped chocolate lollipop by one of my housemates who hid them in the house for us to find today because she's currently away. I spent the day in the library but wore bright lipstick and did my hair for myself. I had a Galentine's Day lunch date with the Sheep. And tonight, I'm hanging out with my best friend, not cooking anything, and having a good laugh. Also she got me a red rose. Friends are the best.
Valentine's Day is about love, yes, but it's up to you to define what love means to you. If you've got a special someone, ey, why not making it about them. Yes, corporations, consumerism, way too expensive chocolate-covered unripe fruits. Whatever. Love, dude. And if that special someone is you, or a friend, or all your friends, or a random someone you meet at a bar, do make the most of it. Today's your excuse.