I guess you're getting the message from lots of folk here to keep her.
You already know about the over-crowding problem in rescues just now, or you wouldn't have volunteered as a fosterer. This situation isn't likely to improve.
Your decision is whether to keep this little soul who has been guided to you, or to put her back in exchange for another little soul who may never find a home, and hope she is spotted in favour of cute tiny kittens, of whom there are hundreds.
Right now, even "adoptable" cats are being overlooked. This is the conundrum we rescuers are dealt.
Every soul deserves a chance - and hers is no less deserving than any other....
Your heart will guide you.
(PS My first two fosters are still with me, and are a pure delight and bonus to our family. Never, ever feel like a failure - whatever you decide - you have helped!)