Transmission slipping gears

Was the ECM replaced due to transmission issues? Was the replacement ECM fully programmed to match your vehicle?
First thing would be to check the transmission fluid level and the color and feel of it. When was the transmission last serviced? Slipping and no reverse and the mileage you list all point at a transmission that needs an overhaul.

I know when the transmission was service last, I bought the vehicle last summer.Thank you for your reply.

Do you mean you do not know or was it serviced when you bought it? Either way I would start with a fluid check and go from there.

I do not know if it was service when I bought it. But I will change the fluid, thank you.

Check it first. It may be full and okay. If it is brown/black and feels gritty then the issue will not be fixed with a fluid change. That would be a sign that the clutch plates inside the transmission are wearing out and the fluid has gotten very hot.
In that situation either a rebuild or replacement is about the only cure.
If however the fluid is clean but low then a fluid change may help.