I wore invisalign 3 years ago, i stopped treatment right before i was done & i made the mistake to not wear a retainer. my teeth have mostly went back. what can i do? do i need to start from scratch?

Probably need to start a new case. Stopping treatment and not wearing your retainer indicates non compliance, meaning you need to have your malocclusion re-assessed. Speak to your treating dentist or orthodontist for possible options.

Your relapse does not result in a carbon copy of your original malocclusion, but instead an entirely new malocclusion. Success of clear aligner therapy depends on an even higher level of patient cooperation than fixed appliance (braces) therapy. Yes, retreatment will be required. See a specialist Orthodontist, but only if you are willing to fully participate.