Have you tried thus?

Pyridium (phenazopyridine) is specifically used to relieve the burning, pain, urgency, and discomfort associated with urinary tract irritation, which often includes the burning sensation while peeing.
It is available over the counter

I'm sorry to hear that. Most post-radiation complications are mild and disappear in a couple of months, but a tiny minority of us get unlucky.

Mine hit about 12 months after radiation. There was a UTI, but when we cleared it with antibiotics, the pain was still there. One time it looked like I was peeing blood

I worked with a urology oncologist: a cystoscopy confirmed that I had radiation cystitis (mild burning) to the bottom of my bladder. I had recently taken a week-long trip and probably didn't drink enough water on the road, which is what caused the UTI to flare up. The solution was

1. Drink lots of water to keep it flushed out.
2. Reduce bladder irritants like caffeine, alcohol, caffeinated drinks, and spicy foods.
3. Take Mybetriq to reduced the bladder urgency, so that I wasn't waking up every half hour at night.
4. Wear pads and briefs if/when necessary.

In about 4 months, the symptoms died down, and I was able to titrate off the Myrbetriq and throw out the pads. It's been only a minor irritation for the past 2½ years. If that hadn't worked, there are more aggressived therapies that the urologist could have offered.

Have you had a diagnosis on this problem yet?

Dr. Rossi mentions in the 2023 mid-year PCRI conference that, if having pain and burning during urination caused by radiation treatments, if they’ve ruled out a UTI and prostatitis, to try these in this order:
> Avoid fruit juice, smoothies, etc. with citric acid.
> Try NSAIDs (Ibuprofen/Alleve/Advil/Naproxin/Motrin).
> Try Prescription NSAIDS (Celebrex)
> Try Medrol dose-packs (steroids)
> Finally, try the over-the-counter product called Azo (it’s basically a urinary analgesic). (@jeffmarc referred to this as Pyridium.)