Well... To "get ahead of yourself" means you're doing something you're not prepared for yet. Sometimes people say as a warning "don't get ahead of yourself!" when someone starts acting upon something that they've yet to finish learning or mastering. It's also said when matters are moving along uncomfortably fast. eg. In business discussions if someone started talking about legalities before the basic terms of agreement were in place, we might say "let's not get ahead of ourselves'. Behind yourself is not an expression I'm familiar with. Are you sure you don't mean "beside oneself"? If so, it's an expression of overwhelming feeling, either positive or negative. "She was beside herself with joy". I hope that is what you're asking.

I wouldn't use "you're behind yourself", but merely "you're behind". It's already clear that you're lagging without describing what you're lagging behind.