Multipage form validation skipped

So I have 5 pages for my form.
And each step one can step through even though there are mandatory fields on the current step.

But off course when submitting the form, the validation stops the submission.
But it is practically impossible to go back and forth to find all those missing values for the fields it is complaining about.

I suggest we do form validation for each element on each step and therefor spare the user from this unhelpful situation.