However after a traumatic experience or during an anxiety state we are sensitised due to shock or stress, the cycle begins of reacting more acutely and the thoughts can become a problem due to the distress they cause, not because they are a sign of madness. Treatment for any intrusive thought is to let yourself have it, don't judge yourself and whatever the 'what if' may be, decide whatever.
Are you a therapist? You write quite assertively, as if you are, but some of your comments seem to be stigmatising mental health issues here- almost saying 'give your head a wobble, get on with it' and suggesting that 'signs of madness' are a bad thing (or something we can control).
In my case, that simply wasn't possible. About six months after our car accident, my husband's life was normal. The accident - to him - was 'something that happened'. My life was not. The accident was still current. I relived it constantly. The intrusive thoughts controlled my life as were having a negative impact on everything. I tried 'letting myself have them', I tried blocking them, but without professional help I couldn't manage them.
I did have a mental health problem. That's ok. If I'd had a problem with my physical health I'd have gone to my gp so I took my mental health problems to see her too. If I'd found a lump on my boob or broken my leg, I wouldn't have 'decided whatever' so I wasn't going to ignore my mental health either. I'm not 'cured' but I have strategies for managing now.
OP, it sounds like your accident was some time ago and is still bothering you. A chat with your gp and a hunt for a better therapist might be a good idea. Good luck 