Hi Ruth, I'm so sorry you are dealing with this too! It's one of my worst issues, well digestive stuff in general too.
I use magnesium oxide as it's known to specifically pull water into the colon. I take 2 types of magnesium daily - one for my bowels and one for my muscles. You can read about
the different types of magnesium here:
www.globalhealingcenter.com/natural-health/types-of-magnesium/ You take it to "bowel tolerance" - meaning you find your dose by starting out on a low dose and increasing it until your bowels are loose, then back down on your dose a bit and see if that is a good amount for you, if not, increase it a bit and see if that will do it. I have to take my magnesium twice a day, first thing in the am and then again with my bedtime pills.
I also do this with Vitamin C, as it will also help move your bowels.
Do be sure that you are taking in enough fiber as well - our bodies need that in order to move the waste along the digestive tract and out of our bodies.
Of course, plenty of water is also necessary, but you should be drinking a lot of water to stay hydrated anyway.
Any time I get a UTI (or other mucus membrane infection), I reach for Goldenseal. It's very effective at dealing with these infections.