I used to have that all the time, depression, stress, it could be depersonalization disorder , or BPD, Pure-O. I don't know why they come , But they will stay if you don't get help. You are probably depressed and that is probably the main cause of these, i had suicidal thoughts for days and days and weeks and weeks and i told people yes i have them but i would never act on them. It's really scary but i can relate to what your going through i hope that you get help. And you'll be fine and once you get help the thoughts will stop.
Obessive thoughts of suicide, whether or not you'd ever act on them, are not normal. If they are scaring you...........for whatever reason, you absolutely MUST see your doctor!
Depression, left untreated, can often lead to thoughts of suicide. You and your doctor need to discuss if this is what is causing your obsessive thoughts. Treatment, with or without medication, but absolutely with therapy, will help you figure out why these thoughts are haunting you, where they came from, what's behind them...........and once you face them, you can begin to heal and recover.
Please see your doctor as soon as possible.
Peace
Greenlydia