How to make the wife more comfortable

I love having my balls busted, but the wife isn't really into it. She does do it sometimes, only because I like it, but she has been going harder lately, but she's afraid of causing permanent damage. I know it's not really the fact that she's hurting me, since I've assured her I want it to hurt, but she really is worried about doing real damage to me. I've tried to tell her it's actually really hard to cause permanent damage, or need hospitalization, but she still really worries about it, that that holds her back alot. I was wondering if anyone had any real life experience/advice to give on how I can assure her more. And how much can the balls actually take? Women's advice would be especially welcome, since they may have more insight. Or if anyone has any articles. I would just show her some ballbusting porn, and how hard they can get hit and still be ok, but she doesn't watch porn.