Update: socializing feral cat 2 months

This is an update post regarding a post I made over two months ago regarding the cat I have decided to name, Baby. For reference, I found Baby at my office and fed her consistently for one month. Around thanksgiving time I trapped and brought her inside as temperatures were getting colder. The first day trapped I took her to the vet and they traumatized her beyond belief and i truly feel that without that visit, she would be much more socialized. Now, she has her own little “apartment”, a pen in a separate room with toys, a bed that works as a covered hide, and access to wet food/ water. She has not one time interacted with my resident cat and my resident cat herself has not had much of a behavior change. The vet was not equipped to handle Baby but I was able to get her a rabies and first round of respiratory vaccines. My boyfriend and I have been so patient, minimal interactions initially, more prolonged periods of just being in the same room but she has had little to no progress getting comfortable with us. It breaks my heart because I know she wants to get comfortable, she’s just scared. She has however loved playing with jingle toys. We cannot get anywhere near her without her hissing and most recently damn she attacked my boyfriend through her enclosure, unprovoked. I feel like a failure and have accepted that we are not going to be able to socialize her much longer and the future of having a snuggly two cat household is not in the cards for the moment. I feel so angry with myself as I will have to release her but the vet said she is too young to get spayed at the moment. Does anyone have any advice? The other emotional aspect as it is projected to be cold the next two weeks.