Why is my dog so hard to train?

Ok, so I have a 4 month old Australian Shepherd and she is really hard to train. I know naturally Aussies are really smart dogs and catch on to things quick. It's not that she isn't smart, because when she figures things out she's spot on. The only problem is she gets really excited and can't focus. All she wants to do is snap at the treats and it's so frustrating because not only is she not learning the tricks efficiently, she is chomping my fingers off! The only commands she knows are the basic sit and down. I try to teach her other things like stay, come, heel, etc. but she is just too much. Please give me some pointers! Thank you :))