Spain
Spain has an abundance of adventure. From bullfights in the streets to world class art, you will not be short on things to do in Spain. Probably most useful, there are reasonable rates for hotels if you're traveling solo. This is something that a lot of countries don't offer so take advantage of it and appreciate it. Spain is a gorgeous country that you will not feel out of place in.
When you travel alone you will learn about the person that you are, maybe it'll even help you become more comfortable in your own skin. Traveling alone will also teach you about the world, or at least the destinations that you travel to. When you're alone you take in your surroundings more than if you had a companion. You can do whatever you want, you can do it at your own pace, there's just no one else that you need to consider. If you're looking for a little me time, consider taking a trip to one of the locations listed.