As for the csa you will have to pay it, they go through income and outgoings, they will take in account that your new partner is pregnant. And how many nights a week you have the child but in your case as you won’t be having the child so you will be expected to pay more than you would if say you had the child one night a week.
As it goes for the rest of it, if your not on the birth certificate you don’t have any legal rights to the child. Are you sure you don’t want anything to do with the child as there are contact centres there in place so you don’t have to deal with the mother and just focus on the child. At the end of the day it’s not the child’s fault he/she was created it’s down to you two and how you want he/she to be brought up. Personally despite what I thought of the mother I would go for joint custody at least the child will know you and know that you never gave up.