01’ corolla transmission death

Hello everyone, I am new to the forum. Thank you for having me.

2001 ex corolla

Unfortunately I am here because this is my last resort. I bought a Toyota Corolla eight months ago and when I was driving on the highway the engine blew out about six months ago.

I replace the engine and the car drove fine for about two months until the transmission blew out.

No this is where my question is, I have already installed (2) used transmissions from the junkyard so far. My mechanic put fresh oil in both, new seals, and said that he installed it correctly. Both transmissions had blown out within 40 miles of driving. Can someone please help me in point me in the right direction please? My mechanic is frustrated and I am frustrated at this point. My last resort is buying another transmission from a working car and putting it on. Can anyone help?

It’s an automatic 3speed

Thank you!