Hello Sleepless, I was also diagnosed with prenatal anxiety and the consultant told me I was in the highest possible risk category for PND (it was my sixth pregnancy, with five losses before hand, which also increases risk of PND.
Obviously the baby needs its Mummy more than any of the added benefits of breastfeeding, and I would take that as my base position and see any breast milk that the baby got as a happy bonus, but not necessary iyswim
FWIW when the baby was born I actually did better mentally than anyone could have forseen and did not need any AD's
The thing that helped me most was some understanding counselling that I started in pregnancy and continued after the baby was born.
It felt like an insurmountable effort to get the baby out and about on public transport so early after the birth, but having a safe space and knowing that appointment was there made such a big difference to me, and I had got used to the counsellor and the journey before the baby, so had already done some groundwork.
Have you been offered any support like this?
Guessing from your username it might help to have a local recommendation- I went to CITY PREGNANCY (can google them) who offer free of charge counselling to women in our situation. You can self refer to them if you want.
Good luck with your pregnancy x