How do I log Instagram out of every device at once? Pretty sure someone else is in my account.

Do this now, in order. 1) Settings > Security > 'Where you're logged in', tap the unfamiliar sessions and log them out. 2) Immediately change your password, this alone boots most other sessions. 3) Turn on two-factor so a password alone isn't enough to get back in. 4) Check the email and phone on the account are still yours, attackers quietly swap those. If the email was changed, use the 'was this you?' link Instagram emailed you to revert it.

Change the password of the EMAIL tied to your Instagram too, not just Instagram. If they're in your email, resetting Instagram does nothing because they'll just reset it right back.