More than one can handle with 荷が重い
力量にくらべて、負担や責任が大き過ぎる。
The burden and responsibility are too great compared to the degree of ability to get things done.
Goo Dictionary
荷が重い is used when one’s responsibilities exceed one’s capable of handling those responsibilities.
A similar idiom in English is to “have too much on one’s plate”.
