How to store data from forms then use it as flow condition

i’m creating my very first app ever, and i’m using flowable and postgree.

my app is basically a student platform that allows people to log in then to see their class schedules.
the accounts are created via a form (flowable form) & the log in is also a form.
i created a choice gate after the start that would send a user to “create an account” if his mail is not already registered

i noticed that all the data i enter via the forms gets stored automatically in a table called “act_hi_varinst”
i want to find a way to store this data, then with the flow condition, compare if the entered mail exists already or not

how would that be possible ? thank you in advance!

here’s my process model (for now i’m ignoing the check wether the password is correct or not)

If the login form in the first task has a field with id “password”, a process instance variable is available to you. Into the exclusive gateway you can use it into a flow condition, such as #{password != null}

For checking if the password is correct respect to your database, you should add a service task making a call to your service for checking username and password.

Hope it helps