You are looking for a technical solution to a user issue. You can choose not to sign into GMail if you really want to. If you choose not to do this, you cannot do what you want. This is a very simple problem with a very simple solution.
Well, this is a Mac Applications forum, not a psychiatrist's office. We can't help you with self-control issues.
You can remove your distracting accounts from Mail. You can quit Mail and remove it from the Dock. You can set Internet activity time limits by account in System Preferences. But a determined human will always be able to get around technology.