I too had/have this problem. I found the Methenamine to be very hard on my stomach in long term care. I was getting UTIs too. One became a more rare bacteria. I started a regimen of natural Oreganol, Olive Leaf, and Activia, Kefir, change the Microbiom. Got a doctor to evaluate the bloodwork and got on a vaginal versa based estriol/progesterone topical. D-Mannose is great for Ecoli but not gram - as much. You can take a lot of D-mannose as it is a common sugar molecule.
When I was taking the Methamine I took a 1/4 dose and it worked. Thats the dose in most of the over the counter remedies. I can't stress a quality Kefir and Activia enough. Good luck.