irregular period not pcos (help!!)

i’m 17 now almost 18 in my senior year, back in sophomore year i finally started getting concerned because i hadn’t ever had a period at that point and i was 15-16 starting to freak out about it, i went to the gyno and they did an ultrasound and there were no cysts or anything odd going on, they put me on medroxyprogesterone and had me take it for 7 days and then my period would start once i finished the pills and have a check up with them about 5-6 months later, well i was getting my period on the meds which was good! however its still not starting on its own like its supposed to, i went to the check up and they said to either stay on it or try birth control, i tried the bc since the first one didn’t work to well. after getting on bc it got worse my period got even more irregular and eventually didn’t come at all for MONTHS. I went back again for a check up and they said to just go back to the original meds, i did, im now getting my period after taking the pills again which is good but after 2 years, of this cycle of check ups, it’s still not starting without the medication. does anyone have an idea of what it could be because we’ve come to the conclusion it’s not pcos. starting to think it could be my thyroid. anything will help thank you!