I did basically the same thing, and began free traveling in my mid-twenties. My first trip was 7 months backpacking in SE Asia. Second was 5 months on a motorcycle in the American West. Third was three months in Ecuador and Colombia. Interspersed between these were many smaller trips.
The three month trip was planned to be a year-long trip circumnavigating South America, but I just couldn't get into the travel thing like I used to. I went out drinking two months into the trip, the night of my 29th birthday, and got really fucked up and acted like an ass. The next day, I couldn't stop reflecting on what my future held. Was I going to do this forever? I mean I wasn't really even enjoying it any more, I was getting old, and my money was running out, so obviously not. I felt like I was just going through the motions: go see something, meet some people, go drinking, repeat. The exciting had become mundane. But if not this, then what? I hadn't given much thought to these questions before, but they became all I could think about.
I met a group of Argentinians traveling in a VW bus, and I spent the last two weeks of my trip riding through Colombia with them. I tried to stop worrying and to just enjoy the moment like I used to, but I couldn't. I ultimately decided that my concerns were justified and that I needed a change. When I got to Bogota, I parted with the Argentinians, hopped a flight home, and enrolled in college full-time.
That was four years ago, and now I'm a senior undergrad in mechanical engineering. Not traveling or living a life of excitement and focusing solely on doing what I had to do has taught me a lot about who I am and why I traveled. I traveled, as you say, to fill a void, but also as a way to run from responsibility. I look back on my traveling days as the best days of my life, and I look forward to being able to travel long-term again some day, but I've changed. That void no longer exists, so I won't travel to fill the void, but to simply enjoy traveling, to relish the freedom it gives.
Are you familiar with the notion that you can't have light without dark, joy without sorrow, sweet without sour, etc? Well, if you stay too long in the light, you become accustomed to it, and it becomes your new norm. It's depressing when things which used to be amazing become commonplace, like you've lost the sweetness in life. And when you go back to the real world, it's even more depressing. The problem is that you've lost your frame of reference between the light and dark sides of life. The reality is that you need the dark to appreciate the light; you need some sour in your life to appreciate the sweet. So my advice is to stop running. Take on some responsibility, and live in the dark for a while. (And by that, I don't mean live in depression. I agree that you should probably see a therapist.) Remember what the 'normal' life is like - actually feel it in your bones. Try to find a balance within that normality, to adjust to it. And once you do, when go traveling again, you'll find that it is as sweet as it ever was.
It sounds like you lack balance. One of the most difficult parts of life is figuring out how to find balance in all things, including yourself. My advice to you is to work towards something challenging and also see a therapist. Focus on something difficult for a while. Don't try to chase the sweetness of travel that is no longer there. Taste the sour a bit, live in it, soak it up. It won't kill you, and will in all likelihood make you a better, stronger person. You'll know when you're ready to travel again. Good luck.