How can one differentiate between striving against the self to perform acts of worship until one becomes accustomed to them, and being overly strict and excessive in them, burdening the self with what it cannot bear, especially since the feeling is the same in both cases?

There is a difference between adhering to the religiously prescribed limit in worship and exceeding it. If you experience a weakening in your resolve to fast on Mondays, Thursdays, and the White Days, then strive against your soul to fast them. As for fasting every day, this is extremism, because it is exceeding the religiously prescribed limit.