Congratulations! You might have OCD. OCD is internal obsessing about things that scare you, in your case suicidal intrusive thoughts scare you and you probably feel like you shouldn't have them. No one can control their thoughts and the more you try the more you fail! It sounds like you reassure yourself that you aren't suicidal (compulsion) this solidifies neural pathways and become habitual thoughts thus making the intrusive thoughts more common. Read up on Pure O - OCD.

Some people with OCD go around thinking they are secretly murderers even though they have never killed anyone and hate their intrusive thoughts of doing so. Some think they are secretly pedophiles even though they don't actually want to assault children. Or some think they are suicidal even though they are sure they don't want to. For these people intrusive thought make their brain freak the F out. Which just triggers more of them, and the reason for that is because our brains are just a bunch of stupid chemicals really.

What makes it harder is OCD obsessions are often taboo. Try telling someone who loves you that you keep worrying that you are suicidal, but you are pretty sure you are not, but the thought keeps popping into your head - they will freak the F out too! Which will make you even more worried that there must be a reason for the thought popping up (there is not any good reason). And so you'll reassure yourself some more, which will make your brain think its an important scary thought and pop it back in there more often.

OCD sucks! But you probably won't commit suicide! So you've got that going for ya.