Oh poor you. It is horrid. However, I am sure you will reap the benefits. You were there for him to reassure him, you hadn't gone anywhere, so he will learn that it is OK to be in there. Make sure you still use lots of rewards when he is in there voluntarily.
When you are trying to eliminate a behaviour it will ALWAYS get worse (peak), before it then gets better and will continue to get better as each time you go through it, that peak will get smaller and smaller.
However, if you reward that behaviour (i.e. crying and whining), by giving in, it will immediately go straight back up to where it was. Why? because random reward is a very strong reinforcer.
Take for example a chocolate machine. If you put a coin in, you get chocolate, if perhaps the machine didn't give you anything, you may think about putting another coin in and if you got chocolate that time, you would then try it again. If however, you put in a coin and nothing comes out, you would try again, if nothing comes a second time, you may try once more, but if nothing came out, you would definately give up..... This is why people do the lottery, because occassionally they win a small sum. If they never won anything or no one won anything ever, they wouldn't do it!
OK learning theory lesson over!! You have done really well, exactly what you should have done. Now you have gone through that pain barrier you may find it easier next time. Just still be around to reassure Storm for a few times, then you can pop into another room for a few minutes and back again!
Ever watched Supernanny?
Well done.
Kate