yourself

You use yourself when the person doing an action and receiving an action are both you.

Example: Be careful. You'll hurt yourself.

Use for emphasis.

Example: Don't pretend you don't know. You yourself have seen these kinds of problems.

If you do something by yourself, you do it alone.

Example: That's beautiful, son. Did you make it by yourself?

If you don't feel yourself, you are different from usual, because you are sick, scared, stressed, etc.