If you Log in with PayPal but have forgotten your email address, then click on the ‘Forgot email?’ link where you can enter up to 3 different email addresses and press the ‘Next’ button. At this point if you now remember your email address, then click on the ‘I’ve remembered my email’ link.
If you Log in with PayPal but have forgotten your password, then click on the ‘Forgot password?’ link where you will enter your email address and will get help creating a new password.