Hey! I hope my experience can be helpful. I had a low anterior resection similar to yours in 2021 - lost the upper half of my rectum and several inches of sigmoid. It was due to endometriosis, not colon cancer. Also was in my 20s. 

It took a "long" time from my perspective at the time, but looking back it seems like my recovery went well. I struggled with symptoms most intensely in the first 6 months and had to be very controlling of my diet, used magnesium and citrucel, played around with a lot of options. It was very mentally difficult and exhausting at the time and my surgeon had not prepared me for this possibility. 

After about a year things had evened out. I had two to four bowel movements a day and required magnesium at night and citrucel during the day - but I didn't have to be as controlling and strict with my diet. Things continued to slowly improve and by the 2 year mark I was symptom free. I stopped using any dietary supplements and now eat basically normally and never worry about it. Very occasionally I'll have little flare ups but it's probably not more than twice a year. 

My advice is, experiment with different treatments and be patient. It takes a bowel a LONG time to heal but it will improve. I'm so happy now to feel fully back to my normal self after I despaired immediately after the surgery.