Letting go of a child you thought was yours...

how does one move on or let go of a baby you went through pregnancy and birth through and was there for everything and at hospital as much as you could and come to find out she wasn’t yours even though you wanted her to be so bad..feeling like your family just died basically you lost two people you loved so much ..how does one let go ?..still love them both very much and the mom did not cheat she also believed the entire time she was my kid and come to find out it could be a guy that might of took advantage of her a night she does not remember..