Auxiliary Verbs – When to Use ‘Am Not’ vs. ‘Do Not’ in a Sentence?

In all your examples only one sentence is correct every time. "Is" is a main verb there. That means we translate it and it is the only verb in the sentence. For example:

What is it?

There is no other verb besides is, and this sentence is correct.

"Do" is not a main verb, it is used to make negatives and questions. There is always another verb in the sentence, which we translate. All the other sentences are examples of that; the verbs are: practice (first example), come (third example), matter (fourth example), care (fifth example), realize (sixth example).

In your first pair

• Let's not is grammatically "correct" and common in all registers; using it in conversation will not make you sound affected or pedantic.
• Let's don't is grammatically "incorrect" but often heard informally; it should not be used in formal writing, but will not expose you to scorn if used in conversation.

In your second pair

• Make us not practice is grammatically acceptable, but would be used only in conversation; and even there something like keep us from practicing would be more usual.
• Make us don't practice is unacceptable in any register.