My teeth are starting to shift years after braces. What should I do?

I got my braces off about four years ago. I wore my retainer religiously for two and a half years after I got them off but after that I stopped wearing it. Since then my wisdom teeth have come in and while there is enough room for them and they grew in straight I think they caused my front tooth to start moving back to how it was before my braces. I tried on my retainer again but it barely fits. How can I fix it without breaking the bank?

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