Is she your best friend? If so she should understand and you should be honest with her. Plain and simple there should not be any stress there if she were a true friend.

I would say something like "Hey, we need to talk, I want to be supportive of your parties and I feel I have been to a degree but honestly I can't come to them every time. It isn't anything personal but I feel that I am taking time and money away from my family and their needs and I really need to buckle down to do some improvements.(make something up, simple but not dramatic enough to make her think your whole world is falling apart and just to get the point) I really do appreciate you including me in your invitations but I just can't do it.

Tell her you are saving money for you guys to do stuff as a family this summer. you want to take your kids some place special.

If it were me in this situation and it were my best friend she wouldn't even say anything except for I understand but just know you are still welcome to come, and end it with a "That's cool"

Honestly I think if she were a true friend she wouldn't expect it from you all the time. I can tell my best friend anything and not feel bad about it.


Edited to say I reread and she is not your bestfriend but I would still lay it out to her.KWIM? You shouldn't have to stress it.