Should we consider rehoming one of our cats?

We have 3 cats, 10y old male, 6y old female, and 4y old male. The one in question os the 6y old female. We've had her since she was a kitten (about 5/6 months). She started off fine but, her behavior quickly changed to be very scared and frightened. She only really comes out when we are sleeping and sometimes when I'm working from home or downstairs watching TV by myself. She is very afraid of my partner and strangers. She spends most of every day under the bed. We have talked about rehoming or surrendering her over the years but I always feel bad because she is a very sweet cat and this is the only home she really knows. We do think she may be happier in a home where she is the only cat. We are conflicted. Looking for advice, thanks.