I already filed my 2018 tax return using an agent. How do I get out of it here on turbotax? When I log in, it keeps taking me to 2018 return process

What do you mean by    "I already filed my 2018 tax return using an agent."   ???

Do you mean you had your return prepared somewhere by a tax professional?   Or do you mean you used online TurboTax Live and had help from a CPA here?

If you have a TurboTax account for 2018:

Many people have multiple TT accounts and forget how to access them.  Log out of the account you are in now.

Account recovery

https://myturbotax.intuit.com/account-recovery/

https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password

Or did you use the desktop version of TurboTax?  If so, the files are on your own hard drive or any backup device you used like a flash drive.

https://ttlc.intuit.com/questions/1901486-how-many-turbotax-accounts-do-i-have

https://ttlc.intuit.com/questions/1901535

"How do I get out of it here....?"     WHAT are you trying to do?

Then what are you trying to do here online if you already filed 2018?  Just abandon Turbo Tax.