I do know what you mean, and with everything going on at the moment it's completely understandable to feel that way.
A friend wears an elastic band around her wrist, and every time she thinks of something in particular, she forces herself to snap it, which hurts, and she is motivated to replace that thought with something else. Hers is for an OCD trait but it might be worth giving it a go?
Edited to add ... sometimes negative thinking comes into play when you feel you have nothing to look forward to (and that's especially the case with lockdowns changing the way we're used to living our lives). Do you feel that way?
If so, plan little things that will bring you enjoyment - they can be as simple as on Wednesday, you're going to look forward to a huge cup of hot chocolate with lots of marshmallows and sprinkles on top ... and on Thursday, you're going to surf the 'net, coming up with a new hobby to immerse yourself in ... just little things that would give you excitement and don't rely on others (who might cancel at the last minute or something). Diarise them, so you can see all that you have to look forward to.