Ineveryone'slife,notonlyonewillmeetvariousdifficultiesthathecan'tsolve【去掉it】.butalsothesedifficultieswillpuzzle【one换成him,避免重复】constantly.
也可以说成:
Ineveryone'slife,onewillnotonlymeetvariousdifficultiesthathecan'tsolve,butalsowillbepuzzledbythesediffcultiesconstantly.
这样notonlybutalso连接的句子主语都是one,更舒服些.