Shyness is ruining my relationships

There is this guy at work who would flirt really hard (it was totally inappropriate) and I began to REALLY like him. 

Unfortunately for me the more I like someone, the more I clam up, act reserved and serious, and just am not myself.

He has backed off now and it really hurts. I have very strong feelings for him but it seems the spark has died a little. I don't know if it's because he thought I wasn't interested or if he himself lost interest. 😥

I don't know why it's so hard for me to be my authentic self, but I just lack self confidence in this way.

The last time I saw him he did give me a hug, but I hugged him the wrong way (with crossed arms, like you hug a friend) and he patted my back (also like a friend hug). 

Is there any way to get back to where we were? I am really hurting because I feel like I screwed up a good thing because I lack confidence.

Also, I will mention that he has been seeming a bit sad and disengaged recently. I told him I was a bit worried about him and he said he's been having some personal problems. (I don't know what they are.) I just thought I should mention that in case that changes your advice at all.