I'm curious, can you provide a link to the design of the antenna you built?
You need an antenna or array with a very good front to back ratio. WCBS is about 6 dB stronger than WFSB. In theory, the signal to noise ratio of the desired signal needs to be better than 15 dB... In the air you're at roughly -6 dB SNR assuming only the signal from WCBS is contributing noise. Given the extremely weak signal levels in the air, I'm sure you have additional sources of noise to factor in... TV Fool has no way to predict those. The bottom line is that your antenna system needs to have 21 dB more gain (as a minimum) in the direction of WFSB than toward WCBS.
Be prepared to patiently and persistently experiment. I'd start with an off the shelf solution. I might try an Antennas Direct DB8E and moderate gain preamp such as the RCA TVPRAMP1R. The forward gain and F/B ratio of the DB8E are at least approaching what may be required.
I would also strongly consider stagger stacking a pair of identical UHF Yagis such as the Antennas Direct 91XG, Winegard HD9095P or Antennacraft MXU-59. If staggered and aimed correctly, the needed F/B ratio may be achieved.
Here is a bit of info re. stacking and ganging,
http://forum.tvfool.com/showthread.php?t=1024. In particular, I'm thinking of the link,
http://www.anarc.org/wtfda/stagger.pdf, page 5
(If you want to try to improve your existing antenna, a reflector would be vital to improving it's F/B ratio and forward gain.)