I think that you are experiencing symptoms of voiding dysfunction due to prostatic hyperplasia. The treatment with Flomax and Proscar is correct, but in most cases, after several years it is not enough, because the prostate can keep on growing and closing more and more the urethra. When this happens, urinating becomes more and more uncomfortable and urinary infections become quite common. Your PSA levels have always been within acceptable limits, so there is no concern about prostate cancer. The fact that you feel improvement with antibiotics means that infection is part of the problem, but the main reason for the repeated infections is probably the enlarged prostate. It would be important to have a urine culture done before taking antibiotics, to choose the best antibiotic for the infection. Now you are taking doxycicline, which is a good treatment for prostatitis, but in a man your age usually the infection is not in the prostate but in the bladder. Perhaps I would recommend to stop taking doxycicline, wait for 5 or 6 days, perform a urine culture and see the result; if the culture is positive you should take another antibiotic.
If your main complaint is pain when you urinate, you should take an anti-inflammatory, such as Ibuprofen.
If your main complaint ir difficulty to pass urine, in spite of Flomax and Proscar, I would recommend a diagnostic workup (ultrasound, flowmetry etc...) and, probably, surgical treatment.