NO! you are not too heavy. YOu are not even as heavy as a normal sized American man, and he would not think twice about being too heavy for a 16 2 hand horse.
the window for losing the fat has not closed. But it will probably never be quite as easy as it was prebabies. Price ya pay. I am pretty overweight now, but when I had my kids, it did take some time before I made up my mind to lose the baby weight, but it happened. then I had antother son and had a bit harder time the second one. Now, well, it's not baby fat, just plain old fat.
hang in there.