You answered it yourself in that you were foolish to let it go so long.

eBay gives you 30 days after the last estimated delivery date to open a case for a refund.

If you used PayPal, they give 180 days to open a case for INAD.

The last option is your bank if you used a credit card (direct or via PP) but they also have a time limit.

Looks like you might be sourcing that part yourself.

Chalk it up to lesson learned.