I let my boyfriend use my Google account because I have two, and now it only signs me on as him even though he signed in on his own phone. Since you can't make a trainer account I was wondering if any of you would know what I could do?! Thanks!
You need to sign out of the account that is logged into Pokémon Go. The associated account is simply used to associate used ID's to the account.
You need to log out of the Google Account he used on your phone, PokeBall -> Settings -> Sign Out, and then create a new trainer with your other account.