As an ADHD-Dad: Why is it so damn hard for me to say no to my adhd-daughter????

It’s not about the most important stuff. I can say no to many things. But when it comes to candy being a specific budget, she always manage to manipulate me to give her a dollar more here and there. We have a strict rule that she can only eat candy on Saturdays which is working well with both kids. But she can easily manipulate me to buy here an extra ice-cream or even an ice-cream in the middle of the week if she has hurt herself in school etc. I’m sick of myself. Is it because RSD? Does anyone of you have this problem?