Hi OP,
I also had PTSD after a difficult birth. Like you it was when I got pregnant again that the anxiety really flared up and I became really anxious about giving birth (and being pregnant generally).
A few things that helped me:
Going through my birth notes from the first time. In my area they call it birth reflections, though it might be something different where you are. I found it surprisingly helpful - like you my memories were really fragmented and patchy, and I couldn't look at pictures/talk about the birth without getting upset. Going through the notes with a consultant midwife helped my reconstruct what happened, check some of my memories, and (though it sounds cheesy) it really switched me from feeling like the whole thing was a failure to feeling very proud of getting through it
Psychologist/counsellor. Saw someone to work through how to deal with panic and how I was feeling about the birth, and do some proper birth planning about different possible scenarios and what was important to me. She was also the one who pushed me to do birth reflections; I was sceptical but so glad she did.
Made a world of difference to my ability to manage the anxiety and panic.
Specialist midwife service. Again don't know if every area has this, but PTSD diagnosis meant I got a designated midwife who I saw more regularly, and same person every time - meant I didn't have to explain the background at every appointment, and had someone who "got" to talk through issues and options with.
Planned c section. Not right for everyone, but was for me - and knowing I had the option of a planned, calm birth helped me through the more panicky moments of pregnancy. Even if you don't go for this, I would say an early conversation with your midwife about what the options are, when decisions would be made, etc, might help you feel more in control
But basically I would say that I am so, so glad I told my GP how anxious I was feeling early on in the pregnancy, because she was great and was a gateway to a lot of good perinatal mental health services in my area that I really didn't know existed beforehand, and which were really helpful. So it is worth talking to someone soon.
Good luck. If it makes you feel better, while it was a hard pregnancy the birth this time round was about a million times better than last time. So it can and does get better, it doesn't have to be like last time  And congratulations!