Keychain password not working on my MacBook

I'm using Keychain to generate all my passwords. I can get my mail on my iPad and iPhone, but it doesn't work in Mail. I have three email accounts. Two of them are working just fine. One is not. The account that is not working is a gmail account. But one of the accounts that IS working is a gmail account too.

I tried going into my Mail account preferences. Before I could do anything, it wanted my Keychain password. I went into Keychain and copied and pasted the password for the account that isn't working. That didn't work. So I tried doing the same thing for the account that IS working. I copied and pasted the password from Keychain. THAT didn't work either.