Fortunately the child is a baby. She won't remember you or be traumatised. The loss, if any, will be yours to carry.
I suspect you've bent over backwards to assure new mum it's an absolute pleasure to look after her baby. You've really gone out on a limb to make sure she knows how you are happy to do this.
New mum has probably convinced herself she's doing you a favour by looking after her dd.
You know, the poor, sad you, who has nothing better to do than look after someone else's baby without pay?
End it now. And be planned and prepared.
Find a parent, family member, friend in need, training course, fluffy dog who now needs your help (the more ficticious the better) and just tell friend it's been a pleasure, but you can no longer look after baby from end of month, due to other responsibilities.
Be prepared that she will offer you money (lower than market rate) because unless she's a moron, she's been peeing over gold dust.
Be prepared that she will offer to make you a godmother as well - RUN FOR THE HILLS - because she's one of those entitled people who exploits other people. Yes, you'll be first up for babysitting every Friday and Saturday.
Just state you're not religious or you've already promised a cousin to be godmother and you think you've got your hands full with your own family members.
Be prepared that she'll exclaim she's in a panic - there's no one else at short notice. Tell her her family or the godmother needs or an agency childminder needs to take over.
Apologoise, but state firmly you've promised your time and attention to a family member.
You don't need to get into a negotiation - you just need to get out and cut all ties.
I honestly don't know how you got yourself in this situation.
It's such a huge thing to do.
But you need to get out of it. You're not respected. You're not valued.
And if you're ever found out?
Just say all the childcare was getting a bit much.
But most importantly find something that benefits you with your day off.
Maybe finding better friends who aren't manipulating and exploiting you.