English Translation
Do it right now, before you forget.
Is this the usted form? If not why would it not end in “olvides”?
The subject of ‘olvide’ here isn’t tú or usted, but the thing you forget. It’s really a passive construction. Think of ‘se te olvide’ as ‘be forgotten by you’. The form ‘olvide’ is 3rd person singular, present subjunctive.