Because DH is the primary breadwinner and does so out of the home, you have fallen into the "default parent" trap in which you are the default parent. When he is home, the children still defer to you . . . and he defers to you. He fails to take the initiative to step in and help. He thinks "wrongly" due to ancient gender norms and societal viewpoints that this is his privilege because he financially supports the household.
However, he got the easy side of things. All he has to do is his job, and he is provided an interruption-free environment to do so.
You, on the other hand, are taking care of children and the home, in an environment that is ever-changing, not consistent, and requires constant attention . . . and all while doing so with a health condition AND while you are studying and training to build skills for a job to contribute income to your household.
Your situation is highly physically and emotionally draining as the default parent, and quite frankly, he is clueless to it, as this has always been assumed to be a "woman's work." But times are different, and should be. Children benefit far more from two active, engaged, patient parents that step in and parent and manage, than merely one parent.
Your entire day and night is spent being constantly reactive to the demands of children, which are unlike adult demands that typically are presented with some understanding of one's constraints to react immediately. You are in the trenches, making decisions without the benefit of sitting quietly in an office and given time to think about it. You are "reactive" 100% of the time. This is hard. This is emotionally exhausting.
Your husband will never understand this if he is not forced to live it. You can begin by explaining how it feels, but he must experience it, himself, in order to gain empathy and understanding. And watching the kids for a few hours a few times a week so that you can concentrate on studies or get out to recharge will NOT do it, especially if it is in the evening when they are tired out from the day's events and all he has to do is feed, bathe them and put them down.
My advice to you is to tell him that you are burned out as the default parent, and must take some time away from the home for yourself. This does not have to be anything expensive or elaborate.
Go visit a friend or family member a few hours train ride away. Take your stuff to study, but plan some fun activities too, and make sure that you are gone for at least 36 hours straight, forcing him to take care of the kids by himself.
And before you make these plans, explain to anyone that he might rely on that you trust (such as your parents or his) that you need him to do this on his own so that he gains empathy, so that he doesn't outsource his parenting to grandparents and fail to experience the pain.
In the future, every time he forgets this lesson, schedule another trip BECAUSE this will be the self-care you must implement as he is not meeting your needs and helping consistently with the children.