Uti problems

Back around the first of may I was hospitalized an was told I had a uti. I had 2 rounds of antibiotics thru my iv & was sent home. Was told to go to the dr a week later. Well life happened an today I was finally able to go to be retested, unfortunately I still have a uti. Does anybody have any idea what could be causing it? My dr told me it could be bc mine an my bf’s fluids just don’t mix. But we don’t do back door to even be getting the bacteria all mixed in. I was sent home with another antibiotic today and was told to take 2 times a day for 10 days and the other 4 pills to take them the day after sex. And I’ll be going back in ab a month to make sure it’s gone. Does anybody have any clue what could be causing it? I’ve been with my partner for 8 yrs an was asked if I could be pregnant or if it could be a sti an both of those are no. I am also on bc.