I think it's good that you are looking at the possibilities: she doesn't care, she cares but doesn't know how to talk about it, or doesn't know it hurts you. You aren't assuming that she doesn't care, which to be honest is what I'd probably do.

There is no way I would let something that bothered me keep happening, mostly because that will make me crazier than anything. Therapists are supposed to be able to validate. They teach that in therapy 101. For me it will usually be something much smaller than that, like failing to praise me for doing a homework assignment or something that seems really trivial when I'm calm. With something as serious as missing appointments or zoning out repeatedly I would expect that there is something serious going on in her life. That's getting into the impaired therapist range.

If you can do it I would try to describe exactly what it was that she was doing: "you missed two appointments in the last month, or I think this is the 4th time I've noticed that you weren't paying attention, what's going on?" Then her response would dictate my response. Like if she said her kid died or something I'd probably say something like "I'm really sorry about that. Do you need to take some time off?" But if she ***** footed around and said that she "had some stuff in her life" or whatever, I'd be more direct and say "it really hurt me when you didn't show up." And if she tried to move on before I was ready I'd bring it up again, and again until I felt validated enough. I think sucking it up would make you more vulnerable to being hurt the next time because each successive hurt that's not dealt with builds on the last one.