The correct understanding of the sentence "Being a mum is hard enough without adding more stress" is:

Understanding two: Being a mum is very hard, so we should not add more stress to ourselves.

This interpretation aligns with the overall meaning of the sentence, which emphasizes that the role of being a mother is already challenging enough, and therefore, it is advisable not to exacerbate the situation by introducing additional stress. The phrase "without adding more stress" suggests that the existing level of stress associated with being a mother is sufficient, and there is no need to increase it further.

On the other hand, Understanding one ("It's impossible not to add more stress when you are a mum") twists the meaning of the sentence, implying that adding more stress is inevitable, which is not what the original sentence conveys. The sentence is actually suggesting the opposite: that we should avoid adding more stress to an already demanding role.