Hello from JustAnswer.
The primary concern at this point would be a prostatitis.
A UTI is commonly used to describe a bladder infection or cystitis, and would virtually always be detected by the urinalysis, so a normal urine test excludes the possibility of a UTI/cystitis, including a complicated UTI. It is also worth noting that young adults only rarely get a UTI/cystitis.
Urethritis is more common in young men, but they would usually respond well to the antibiotics that you have already taken. A prostatitis is also fairly common in young men, and it is more common for a prostatitis to respond poorly or slowly to antibiotics. The Bactrim may help a prostatitis, but it may be slow to respond.
So, in this situation, a prostatitis would be the primary concern.
There also are some non-infectious conditions that can mimic a UTI, such as interstitial cystitis, but this is much less common and would usually only be considered once infectious causes have been excluded.
Since your symptoms are persistent, then it would be appropriate to pursue further evaluation, particularly for a prostatitis.
If I can provide any additional information, please let me know.