If your account is connected with Facebook, it uses your Facebook e-mail address. To change that, you'll have to change your e-mail address through the Facebook account settings page.
Otherwise, if you're sure you forgot your password, I would suggest you to get in touch with Spotify. Explain your problem and they will be able to help!
You may get in touch with Spotify via their Twitter page (@SpotifyCares), or by following this link (you won't have to login with this link) to send an e-mail. If you send an e-mail, please check your inbox (and spam box) to check for their automatic reply. Once you get it, please reply to it immediatly (even if it's from an no-reply e-mail addres) to make sure an Spotify employee will get back to you as soon as possible.
Spotify aims to reply within 24 hours.
Please keep in mind that they probably ask for some private information like date of birth and the last 4 digits of your credit card to verify that you're the owner of the account.