I have some questions. Do you know what your Ejection Fraction (heart pumping efficiency) is? Did the operative report state that the damaged heart cells could be revived? Coreg is the drug of choice to revive those cells.
Coreg is considered by some heart patients, me included as a wonder drug, Many, many people who take the drug have seen an increase in weight when it is administered, me included. However, it helped my heart health a great deal, the the weight gain was temporary. I could see that it was lowering my heart rate, so of course the metabolism is lowered, and weight could accumulate. I raised my exercise level and was very careful with my diet, and the weight came off. I agree with Kenkeith, you may have some heart failure issues that mean you are gaining weight because of fluid accumulation.
I think you are making a big mistake quitting Coreg and not telling your doctor. One of my closest friends has heart issues like me quit Coreg because of weight gain, and had quadruple bypass surgery in two weeks.
Consider my post, and do keep us informed.
Coreg helps regulate a normal heartbeat and lower blood pressure. A reduction to 3.125 twice a day is lowest therapeutic value, and the medication should not be discontinued without doctor's advice. If you had a minor heart attack, you may need medication. I don't know of any weight gain associated with coreg, the reduction of coreg may be for other reasons other than weight gain.