Single life is brilliant. I'm sorry you're sad. But nothing is forever. Sad is not forever. Focusing on being single until you are happy being single is a very good plan. You might find you like it better than being in a relationship when you get the hang of it.
Think about things you like to do. Think about things you wish you could do. Think about places you like being. Think about people who, when you're around them, you feel good. This is your new project. You are your new project. And, like with anything new, you might not be that good at it to begin with. But you will get better at it.
Do everything for yourself that you would like a partner to do for you. Breakfast in bed. A surprise weekend away. Little thoughtful presents. Whatever floats your boat. Float your own boat. Learn stuff. What do you wish you could do? Learn it. There's tons of stuff available online now. You could start learning something tomorrow.
Treat yourself like royalty. After a while, you'll get used to it, and you'll expect others to treat you with the utmost respect too. And that's self respect sorted.
Having a partner suddenly leave you doesn't mean you are bad at relationships. It means they are bad at leaving relationships, and bad at sharing their feelings. Nobody should be expected to just intuit that their partner is about to leave them with no warning. She has treated you appallingly. Don't blame yourself for that.
You will be ok. Heartbreak is shit. There's no two ways about it. You're having the absolutely normal response to the way you've been treated. Your life will seem more worthwhile when you put worthwhile things in it, but do it at your own pace. If you want to have some time off from doing stuff to just rest and recover, that's ok. Life doesn't have to have a point all the time. Sometimes you can just be. Emotional recovery happens when you're just being.
You say you haven't got anyone, but you have you. Make that a valuable relationship.