Trouble emptying bladder

I have been getting recurrent UTI's ever since I got married almost 2 years ago (became sexually active). I developed the perfect routine of showering before and after, peeing before and after, no soaps down there, taking d-mannose and generally drinking more water. It improved but it still happens even all this time later. So I had a cystoscopy today and they said I'm not emptying my bladder properly, which makes sense now as usually when I go empty what I think is a full bladder, I always feel the need to pee again 5 minutes later even though before I felt I had completely emptied. So they said this could potentially by why due to urine laying stagnant in my bladder. Obviously I'm trying to be UTI free and I read not emptying properly can be due to weak muscles but have also read could be due to narrow urethra therefore you wouldn't want to tighten the muscles to make it thinner right? Anyone had any experience in this? Does anyone have any advice for me or any UTI advice in general. Thank you!