Wow, what a nightmare! I am so sorry that you have had such a worst-case experience with brachytherapy. The vast majority of men who choose this treatment do just fine, with minimal side effects.
Anything can happen, as readers of the fine print on medication information sheets can attest. I hope that you can get some follow up medical treatment that addresses the problems you are experiencing.
By the way, welcome to the forum. There are some helpful folks here and a variety of experiences with all kinds of treatments--older, newer, and investigational--and side effects.