I think you are ruminating and so anxious about these things that it is just giving you even more thoughts. It is hard because outright denying paranoid thinking can make it worse, but I am a stranger on the internet telling you I've had similar thinking and it actually means you're even less likely to do it. You don't want to do it, so trust yourself! And if you can't a therapist and even meds may really help you.

You need to talk to a doctor AND therapist about everything. You can't excessively contact near strangers (your old classmates) which is inappropriate and intrusive for them. You need to get actual help so you can overcome and manage these thoughts, maybe even completely get rid of them. They will teach you how you don't need to embrace them and believe them OR completely reject and fear them, and that will actually make them a lot less frequent and distressing for you. You can also talk to a doctor and rule out the mental illnesses that you probably don't have like the mood and personality disorders (they have very specific criteria and it's likely you don't meet them).