Why do I feel like urinating immediately after I pass urine?

Hi doctor,

I am a 36 year old, active male. My height is 6 feet and weight is 175 lbs. Previously, I had Zoloft for anxiety. Now, I am just doing exercise, eating healthy food and vitamins. I have been diagnosed with prostatitis a few times in the past few months. I was also diagnosed with this roughly three years ago. The symptom includes feeling like urine stays on the tip of my penis, thus the constant feeling of the need to urinate. There is no real pain, just discomfort and I have always had a problem with hesitation and fully emptying my bladder. I have taken Bactrim for three to four weeks and now, I am on a six week cycle. I have gotten better and it came back twice during this cycle. I literally have the feeling that I need to urinate right after I urinate. I had a prostate examination three weeks ago and said that it was slightly enlarged, but not concerning. I had an ultrasound of my bladder and emptied almost all, but a few left in. My urine was tested for infection and came back negative. PSA done and there is no cancer. I am looking for advice. Thank you.