Recently, I’ve been trying to WILD as opposed to just having dream induced lucid dreams, which is what I usually have. Unfortunately, I keep having the same problem, which is that I feel like someone is sitting on my chest, making it hard for me to breathe. At this point, I can feel that I’m just a step away from falling asleep while I’m still conscious, and having a nice WILD, but it’s sort of like falling of a cliff for me; I know I’m not supposed to keep falling and hitting the ground, but I just can’t stop. Every time this happens, I have to take deep breaths, which breaks the numbness and forces my body back into wakefulness. Can anyone help me with this? 
Are you sure your not just getting excited or trying to hard? WILD is a tricky thing. You can usually enter WILD if you can manage not to move for a long time. Never heard this kind of problem, but hope you fix it.
WILD is the most difficult technique to master but the ultimate of the lucid techniques, don’t give up!
This happens because your unconscious is gaining control over your breathing. When you feel like someone’s sitting on your chest you just have to try and ignore it. Don’t worry, your unconscious will breathe for you. Focusing on breathing will help wake you up, since you are focusing on your physical body. You should try and forget about your physical body, focus on the dream. If you see HI, focus on that.
It might be a bit hard to ignore this problem, but you’ll be able to do it eventually. Remember to tell yourself you don’t have to worry about breathing, the same way you don’t think about breathing while your in a ND, for example. 
Ok, thanks for the advice, I’ll try that tonight (and I won’t give up! 
)
from what i heard also it might be because you conscious mind is trying to make scene of what it has never felt before. that the reason it is harder to breath is because you are going into SP and your rip cage is starting to move less and seem to get heaver [at least to your mind it is]. i might not be 100% right on that but that is what i have read before and heard before.