Trying to introduce two fosters and one is being a bully and making the sweet friendly one very scared. It makes me want to give up.

I've got two foster cats at the moment. One I've had since the first of December (Molly) and one I got last week (Annie). Molly is a sweet girl but has never been tested with other cats, and that, plus her age (4) means she's getting no adoption interest. She's an incredibly playful cat so I wanted to see if she'd get along with another cat so she had somebody to play with. I got my second foster Annie last week in the hope that they could be friends, as she's come from a situation where she lived with multiple other cats. I've been sight and scent switching and feeding them through the door until Molly stopped hissing and I thought we were ready for an introduction. Poor Annie walked right up to Molly for a sniff and Molly really lashed out at Annie and wasn't distracted by play or toys or anything. Poor little Annie is now hiding under the couch and won't come out, and I feel soooo guilty. She's such a beautiful little thing and I don't want this introduction experience to traumatise her against other cats. Should I just quit? I think Molly really would like a cat friend to play with but it's not worth sacrificing Annie's confidence. I don't know where to go from here because I feel so guilty. Any suggestions would be welcome.