Yaya, I can totally understand where you are coming from. The exact same thing happens here. As soon as ds starts doing a bit better with sleep training, he gets a cold. He currently has one now, so we are in a holding pattern (I am still WO when I say good night right now, but if DS puts up any fuss, I give to a GW.) But that is how it has been for us for months now, very, very frustrating! And I know you also probably have the added stress of more of a time line, which doesn't help. Just try and be patient.
One thing I have done that I have seen that helps, if the cold isn't horrible (w/ my DS I think it's actually teeth causing it, so he's just so snotty), instead of taking him from his bed, I stay with him, hold his hand, do everything in my power to make him comfortable there... it helps in the long run to try and keep them in their bed as much as possible. Don't get me wrong, I have completely given up and gotten DS out and had to start over. Of course this is much more tiring for you, so maybe with your exhaustion, it might be good just to hold and start when everyone is fresh and feeling better.
Just to let you know, it may feel like it will be as hard to start again - to you, always was for me at least, but my DS is always a bit (surprisingly

) better the next time around.
I hope that helps you at least a little. Looks like our LO's are about the same age, my DS is almost 18 months.