@frankie sorry to hear about your skin, it sounds really uncomfortable.
A friend who had been through it suggested putting the cream on and then putting glad wrap over the breast etc during the course of radiation and before putting on a bra. I don’t know if it helped, my skin was very red and a bit itchy but it didn’t break and was more like bad sunburn, so perhaps not as challenging your condition. My nurse added a cream with a small amount of lignacaine towards the end of the treatment and yes, similar to you, towards my armpit was where it was a bit more sensitive.
It took about 3-4 weeks to settle down. All the best.