I also had a lot of anxiety leading up to DDs first birthday, so I can relate to this. I think it is very common for women who have had traumatic birth experiences to feel anxiety around their child's first birthday.
I did a lot of therapy, and that helped.
The party planning became part of my healing process, too. We planned a huge party with family and friends, and it was actually helpful for me to take all my nervous energy and channel it towards something productive. I made sure there were a lot of people at that party who understood what I had come through during the past year, and so I used it as an opportunity to get a lot of support from my community. That approach might not work for everyone, but for me it was helpful.
I got in a huge fight with my partner about 4 days before her birthday. That was painful, but in the wee hours of the morning we finally had a very raw, but very real conversation about some things that needed to be aired. Afterwards, I felt like I could move on from some things that had kept me "stuck" since the birth.
For me, getting through the first anniversary of her birth was a huge deal. There were things that I just couldn't let go of until her birthday had come and gone. It was like I needed to go through an entire cycle of the seasons before I could really begin to have new perspective. I wish that I had been able to heal faster, but for me it took a year before I could start to let go of the trauma.
It is nice to now be approaching DDs 2nd birthday and to feel much more "normal"...not like the whole thing is freighted with anxiety & significance. I can remember feeling traumatized, but I'm not carrying it around with me all the time like a huge weight.
The journey is different for everyone. I wish you well as you find your own way.