More than one can handle with 荷が重い
The expression 荷が重
A similar idiom in English is to “have too much on one’s plate”.
Dictionary Definition
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
Examples
Too much to handle on top of class representative duties with 荷が重い

Takako and Azu tell their teacher they want to participate in the sports day relay race. The teacher points out that participants have already been decided, then one of the decided participants chimes in.
- 生徒
( : - 「私
( もクラス代表( はちょっと荷( が重( いなと思( っていたので…」- “I was also thinking I have too much on my plate as class representative.”
- 「できれば代
( わってもらえたら嬉( しいです」- “I'd be happy if I could switch out.”
- “I was also thinking I have too much on my plate as class representative.”