Because your priorities extend beyond you in the form of the life you brought to the world. In short, your kids occupy a huge part of your time, priorities and goals etc. And it's not an inherently bad thing because all of these obligations do give you the opportunity to become a better person and a more responsible adult.
But it is very important to realise that if you keep working incessantly, it will always push you further to the verge of burnout and no one wants that. It is very important for you as a mother to always try to rest and recuperate so that these responsibilities don't take a toll on your physical or mental health.
When we talk about rest and mental health days. It is very important for us to have a healthy outlook towards these things. Because resting is not taking a one day trip or a weekend trip to some resort or spa. Resting is gradua, resting is consistent. You should rest no matter where you are. A very important way to make sure you are resting enough is to be self disciplined when it comes to you and your child's sleep schedule. According to studies, people with a better sleep schedule not only deal with stress better but are also more productive throughout the course of the day.
Asking for help is something most of us struggle with. But as a mother you have to juggle through so much. And which time does not get any easier so it is very important for you to make sure that you have some helping hands around you. If not family and friends you can always opt for a housekeeper.
Remember: you are a great mother no matter what there's nothing such as a bad mother. So don't be too hard on yourself.