How to avoid injuries from long time use?

So, I've been using chastity for a few months by now, tried to decrease the size until the flat in using, but always developed an injury underneath my balls. I always notice when it's about to come, because I can feel a burning sensation in the area. I try to use some lube or cream to prevent it, since some subs told that's what they use to help the injuries to heal

My Dom is worried about these injuries, and, at first, I thought they're due to the size of the ring, but anything bigger would make the chastity easily go off, which isn't against the whole idea of the device

I started to suspect that it might be due to my penis size (13.5 cm), which is not great, but it's thick, and we've been playing with smaller cages (which look hot)

We've talked before and my Dom considered that he doesn't want me to not have erections anymore, because I could be of use to him and his anal pleasure, so I approached him with the idea of using the smaller cages occasionally, while using a bigger one daily (so he can have this control over me)

He accepted the idea, however, I'm left wondering what could make those injuries happen, and how to prevent them

Btw, I don't I was clear before, but each time I develop an injury, I was ordered to talk the cage off and heal before using it again (like I'm doing it right now)