Ugh, I hate those situations too! What the poster above said about it getting easier the more you say no was really true for me. Some things that helped me put an end to that stuff:

I told people that I was cutting out coffee and lunch dates completely because I was wasting too much time and money. I said that I found it hard to say no when people asked, so I was just going to say no to everyone all the time.

When people press for my schedule, I just tell them that I can't remember. "I know I had a bunch of stuff going on this week/that day, but I can't remember. I'll have to check when I get home." A relative of mine does that kind of phishing and it drives me nuts. Sometimes people are extremely persistent and it is very hard not to accidentally give them some piece of info that they then latch onto.

If you chose the restaurant, would it make a difference with this person? If you think about telling her that you can only see her once a month, do you still feel the dread?