Hi Dave,
Sorry for what you are going through. I had similar radiation nearly 25 yrs. ago for post surgery back up after endometrial Cancer. Surgery took care of tumor that was only half way thru uterine wall. Dr.s wanted to do follow up radiation like you did, and they read me the statistics. The Cancer was aggressive. I was 50 yrs. old and naïve about all this. I was limited where I could go due to insurance. The Drs. spoke so lightly if at all about side effects. Oncologist also went for additional radiation (vaginal) low dose in hospital. This was questionable in retrospect, but I trusted as I had little support and never had major health problems previously. It was scary. I had to trust.

I developed IBS. a couple years after the radiation, and still have it today. I see a GI Dr. Went to Hopkins, had all kinds of tests and colonoscopes. I took the Rifaximin $$$$$$ last Spring, and it may have helped, but mostly made me more constipated. I handle constipation by using Mirilax adjusting the dosage as needed based on daily results. Some take it in evening, and some take in first thing in the morning. I also follow a low Fodmap diet. This all helps.
Like you, I get angry. I get angry because they should had gone into more detail about the side effects. I did not have the internet in 1997. I will never know if I would be alive today had I opted out of these extensive treatments. I only know Cancer survivors live in constant fear of recurrence, so at least I know I listened and did all I could to survive ( 50 is still young and there is still time for recurrence. My GYN said that he would not have recommended radiation if I were in my 70's at the time.
If the constipation is severe, the Dr. may recommend a drug called "Linzess". Its pretty powerful, so I would go slow easing into it. It was too harsh for me, but many like it. I believe that radiation also messes with the microbiome, but GI Drs. don't address that well, as they come up with the standard off the shelf probiotic. Some will cause bloat as per my experience. Going to try it again soon in a more customized version.
A functional med or Naturopath might be a big help, if you have the $$$$$, but be careful as too many greedy people are hopping on that train. I started with one, but too much$$$$$$$$$$.

I wonder how much scar tissue factors into the intestinal problem. I had a hysterotomy, then appendectomy 12 yrs. later. There is likely scar tissue, and I am considering a viseral massage therapist ( a little tricky to find). I wish you the best on your journey, and don't beat your self up for doing the radiation. You had every reason to believe it was best, and it may well have been the best choice. I had the radiation at 50, and am now 77 and healthy except for the damn IBS. A lot more people than we realize have digestive woes. Very frustrating! Arm yourself well with questions when you see your Dr. Don't let them blow you off or tell you to just eat more fiber! Focus on what you have and not what you lost! Best Wishes. Oh, PS: don't be afraid to fire your GI Doc if you don't resonate with him(her). Our previous experience makes us a little more discriminating!