I'm the same age as you and although this year has been quite good for me in many ways, I've felt a similar way to you for a while and lots of my friends do as well.
For many people, the early to mid twenties are a period of flux. Lots of us are trying to forge a path in life, finishing university and looking for careers, relationships and the kinds of emotional and financial stability that are hard to maintain at our age. As the years go by, lots of these issues get cleared up for many people, so the underlying instabilities that cause this way of thinking stop being such an issue. I think the important thing is to feel that you are working towards something. Those of us who go to university have an obvious goal while we are there and I think it's important to keep striving for things afterwards. For example, I've just graduated and I am working towards postgraduate study, so there is something that can occupy me for a while. For plenty of friends who don't have anything lined up, it feels that they're walking into the unknown, which is a very scary feeling indeed. I think that feeling of being adrift in the wider world causes problems for a lot of people in their early twenties but with time and experience, it gets easier.