You are wired to think that you currently exist and that you should keep doing that at all costs, but those beliefs cannot be explained or comprehended in terms of logic, so for all purposes they are false, and acknowledging that has been useful to me.

Anyway, if your existential crises are very short and intense, accompanied by intense fear and feelings of impending doom, feelings that you can't breathe, then they probably have a physical component to them. It is very important that you understand the difference between regular intrusive thoughts of death and panic attacks that express themselves as feelings and thoughts of death that you might already have interiorized. The former can be managed, the latter cannot, and are usually self-reinforcing and need a combined therapy.

If you think you might be having the latter (I did for a couple of months), avoid alcohol for a while, and talk to a doctor; you may be advised to take a low dose short-acting anxiolytic drug whenever you feel like you're going to have one (I started with 0.5 mg lorazepam sublingual, then switched to 0.25 mg).