Wait I’m confused. You don’t “love” your ex anymore yet you laid down with her and made a baby?

Not having sex with a girl you're breaking up with would have prevented this.

Just explain the situation to your new girlfriend. She should already be aware that contraception fails sometimes.

Be completely honest with her and be prepared that she will be very pissed off at you. Accept that and tell her that you need to do the right thing by the baby. Be responsible, let your ex know that you, being a mature man, will always help raising this child. This is not the baby's fault. Don't you or anyone try to punish this child over your mistake. I was in a similar situation many years ago. Becoming a dad turned out to be the greatest experience of my life, even under very difficult circumstances. Good luck.

Breakup sex is pretty common but... Nothing is 100% preventative. Always. Cover. Up. Hard lesson to learn but hopefully the kid doesn't suffer for it.

I’m guessing she was on top when you did it. Because in both cases, you ****** up.

What the hell kind of birth control is an arm implant??? Regardless, A) you were breaking up so you shouldn't have had sex with her again anyway, and B) that's a weird form of birth control so you should have also used a condom.