What can I do about my teeth going back to their original placement before braces because of not wearing retainers?

I had gotten my braces off at the age of 12 with only wearing them for a year. My teeth were actually very bad but the orthodontist insisted that a year was enough. They were straight though , well at least for the time being. I was given Invisalign retainers and told to wear the each night. Being that young , I didn't really stick to wearing them each night after 2 months of getting my braces off. Now 4 years later my teeth are going back to their original postion. What can I do about this ?

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