How do I get rid of this anxiety disorder?

I think I have OCD. Whenever I do something I have to double check that I have done it and if I feel relaxed a second later my mind tells me to feel cautious again and that I need to talk care of something or do something. An example is yesterday I just took an exam where I had to fill in my candidate number and name on the cover sheet. After the exam I kept worrying that I might have filled in the information wrong and I couldn't feel relaxed even though the exam was over, which I was supposed to be happy about.

Now all the final exams are over, it means that my summer has started and I have plenty of time to do basically nothing. But this anxiety tells me that I have something/tasks to do but I've forgotten of. I just want to feel secure and relaxed. :(