I'm paying for my fiance's son's insurance with no help

I'm pissed. So my job offers really good benefits so I have me and my fiance on the policy. Well. His ex was afraid she would lose her medicaid that provides insurance for their son, so I put him on my insurance. She promised to pay me $100 a month to cover her half off what it costs to add him a month. She is now refusing to pay and my open enrollment is closed. My fiance told me he cant make her do anything and told me that I need to talk to her about it if I have a problem. so I basically feel used and like I got screwed over. I love their son, but it is not my responsibility and I'm so angry that it is all falling on me.

Am I overreacting? I dont mean to sound selfish, but this is not the first instance of me being put in this kind of position with their son.