You can’t logic away your feelings.

It seems to me your problem is not so much having “negative” thoughts, but the feelings that underly these thoughts. If you’re constantly asking for reassurance, those feelings may be fear, anxiety, guilt? Your thoughts may prompt these feelings, but they also arise from these feelings. Simply trying to over-write the thoughts may not be working for this reason.

Let’s pretend, as a metaphor, that your feelings are small creatures that give you thoughts to think and want something from you. Perhaps paying closer, quiet attention to the feelings themselves, without trying to “get rid” or piling too much thinking on top of them, will help these “creatures” feel heard and appreciated and they’ll have less need to be so noisy. That means, once you learn to be with the feeling, the thoughts will tune out by themselves.

Or maybe there’s some stuff going on in your life that just makes you really, really worried for legitimate reasons. If that’s the case, no amount of positive thinking or reassurance will make those worries go away. Pay attention to what it is you fear and thinking of ways to increase your sense of safety and security. That might be helpful.

I hope you find peace and safety. My best wishes.