Please try to stay calm. Baby won’t be harmed - remember women give birth in prison / during air raids / in the middle of floods / other stressful circumstances and the babies are fine.
Its you who will suffer from all this worry. have you told your midwife how stressed you are?
You don’t have to do ANYTHING about your ex right now, just ignore the letter. Have you blocked him on platforms except email? If not then do so now.
Make plans for your birth. WITHOUT him.
Do you have a female friend or relative who will be your birth partner ( ideally someone calm who has given birth herself ) ? If not , ask someone you trust now.
Have you bought the essentials, such as some baby grows, nappies , a car seat and somewhere for baby to sleep ? If not then do so.
Have you chosen names for baby?
You need to make all your plans for yourself without any reference to him whatsoever. DO WHAT YOU WANT, it’s not about him, it’s about YOU and BABY.
Baby won’t care about anyone except you - they are programmed to bond with you first, that’s how they work. All baby wants is to eat, sleep, be comfortable and be near you.
Amd it’s your job to care for baby and for yourself . Your ex wants are no longer your concern.
You are getting all worked up trying to work out what he wants and why he wants it. You need to let that all go. You can’t make him into someone he is not by trying to pander to him.
I know it’s hard. in your head you had imagined this cute little ideal family . You need to accept that’s not going to happen. You will be a single mum ( like me and half the other women on MN) and you are going to be great !