Btw, I have something that happened when a lot younger that I know effects me now and is probably the cause of a lot of current problems - not directly but in leaving me defenceless and vulnerable to stuff.
I've had counselling and finding it very helpful, and haven't as yet mentioned the first awful thing. I haven't told them it exists and I guess there's enough to work on without that, and I was worried that is have to tell all or sod off, but the reality is not like that.
It is different from your case where it sounds like there has been one awful, horrific thing happen, but I thought I'd share in case it's helpful.
I have realised through the counselling that I go from one extreme to the other. From being terribly good at blocking out and forgetting - to a bad extent where it scares me and I want to 'learn how to remember' but then also having really bad flashbacks and intrusive thoughts.
My counsellor is a great believer in NOT telling stuff unless it's right for you at that moment. Denial and dissociation can be our body/minds protection and forcing it can make us flooded and unable to cope.
But also the flashbacks and traumatic stuff that slips through the net, well, that's because stuffing into a dark wooden box at the back of your brain doesn't help long term.
The flashbacks etc can be because you haven't managed to process it yet, so in a way you're experiencing the raw pain of those moments over and over again... So sometimes by dealing with them very gently or getting at them via other memories or experiences, that can help slowly get some space in between you and the awfulness.
I would try your GP again and ask for a referral to counselling, it's not their job to diagnose or block you and you don't have to tell the GP you can't talk about it. You could say that you're not sure if you'd be able to BUT you do know you'd be able to talk about a lot of the stuff around it/ the consequences of it. And point out that a therapeutic relationship will change what you will be able to say over time so it's not fair to write you off without even the chance to try.
Basically, I think building a relationship with a stonking good counsellor would be really beneficial for you, and they could help you work it through at your pace and help you examine the reasons and terror you have about 'telling'... All without pressuring you to share.