I don't have any great advice to give you, but I wanted to tell you not to be so hard on yourself. You have the right to be feeling all the things you are feeling, but you shouldn't feel shitty, you're not MAKING him feeling angry and sad, you are doing your best with a situation you didn't ask for or expect.
You're not an "inexperienced little girl" if you don't want to be poly, or don't know if you poly could work for you. What I might focus on now is reading some polyamory books (both of you should read the same one(s) and maybe discuss a chapter at a time to see just where you stand on the ideas in them), keep reading forums, and take it slow. Maybe you might be open to trying a mono-poly relationship if it seems he wants to date other people and you don't, and maybe that just isn't for you. At least it doesn't seem like he's desiring to rush into anything, so you both have plenty of time to think and discuss the options.
good luck. *hug*
Happiness will never come to those who fail to appreciate what they already have.