I can't wrap my tongue around it

In a whirlwind week I've gone from expectedly single to fooling around with someone to having a boyfriend. boy.....friend...the thought of it is all too much. I'm just letting things flow right now, how can I tell him that I have doubts? He seems so sure of it all, so sure that I am some amazing creature. I know I'm not. I'm scared.....in the end that's the truth.