Do you have OCD, by chance? A large symptom of that disorder is intrusive thoughts, and they hijacked my life for a couple of years. It was horrible, until I received the proper therapy in order to deal with them. Look up ERP (Exposure Response Prevention). The principle of it is basically this: It doesn't matter what thought you have, but how you react to it. If you react badly to having thoughts of killing yourself, then of course they will distress you, and the thoughts will probably keep coming back STRONGER. But if someone has a thought of killing themselves, isn't stressed out by it, then the thought probably won't come back often, if at all.

The principle works for any thought, such as someone thinking about being in a car crash. If they keep stressing out about it, the thoughts will probably come back bigger and scarier. The trick is to expose yourself to these thoughts, little by little, until they no longer make you feel anxious.

That being said, intrusive thoughts can be a part of any disorder, I think, not just OCD. Best of luck to you in finding out what works for you.