Two natural approaches for me is to take magnisium citrate, 150 mg in the morning, 300 mg in the evening, regularly. But I started slow to see how my body would respond. I'd suggest taking 150 mg in the evening for a few days, then add 150 mg in the morning for a few days, then increase more if need in the evening first, then morning, etc.
The other is to take vitamin C throughout the day to "bowel tolerance". I use vitamin C (Buffered Sodium Ascorbate Vitamin C Crystalline Powder). A 1/4 teaspoon is 1.25 grams (or 1,250 mg). I worked up to 3 grams a day with no "rumbling tummy", but as I worked my way higher, things began to start moving.
After that I dropped back down to 1,500 to 3,000 mg a day.
If you're using fiber supplements, I'd encourage you to eat vegetables instead. I think fiber is best consumed in it's whole, natural food form. It's good that you're drinking plenty of fluids each day. Hope some of it is water.
Hoping you find what works best for you through trial and error. I have inflammatory bowel disease, and am experiencing a mild flare right now, so am going to try to fast for 3 days to rest the colon. Am also drinking lots of water. It's not fun when we're not well.
Edited to add: EEK! I forgot! Higher doses of vitamin C will cause your glucose meter to read much higher. Because you're a type 1 diabetic, using vitamin C could be very problematic. Don't do it! Apologies!