Apologies in advance, I know nothing about cars. My car is a 2000 Civic with 96k

I have had a series of check engine light problems. My CEL came on approx. 15 months ago, so they recommended I get my "cat" replaced. The aftermarket brand was cheaper, so I went with that. Then about 7 months later, the light came on again, and my mechanic said it was the cat but it was under warranty so I got it replaced again.

Just recently my CEL came on AGAIN. I went to autozone and they gave me the code, saying it's p0420. What should be my next course of action? Are aftermarket cats that crappy? If I cough up a ton of money to get the oe cat, will this actually fix my problem or just postpone it? Huge TIA. I don't have much money to fix this and it's stressing me out.