meaning in context - Have to / having to?

As fas as I know, "have to" is the commoner version of the two,
but I'm finding more and more that "having to" is also used instead of "have to".

She has to / is having to look after herself now.

Are both freely interchangeable without any difference in meaning?
If any,I want to know what is the difference between the two and when you use "having to" more preferably than the other.

Proper contexts with good examples to be given will be very helpful to me.