It’s probably a parasite that exploited your despair, I would guess. As Mulberry mentioned - selling a soul is not a thing, but giving such an agreement is an invitation for lesser beings to jump on. I have such issues myself.
There is a wealth of information on how to fight parasites in the “Parasites 101” threads and other relating to parasites and techniques to get rid of such.
I, personally, would advise not panicking, staying calm and in control of yourself, giving yourself assurance that you are safe. (They do feed on negative emotions)
If you want a quick and beginner friendly starting point, I personally benefitted from angelic-words-of-power-and-mantras and earlier on Robert Bruce’s Practical Psychic Self-Defense Handbook has a core affirmation (that you may repeat both out loud and in your mind):
“I am loved and I am worthy. I am safe and I am free. I am powerfully protected. I am master of my body and ruler of my mind”
It’s not as bad as it seems.