Can a lower uti go away on its own? it doesnt feel like i have a bladder infection. and would having urosepsis in the past make my bladder more vulnerable to infection and are antibiotics really necessary ?

Yes, it may. You should drink a lot of fluid in the next 24 hours as a trial. A total fluid intake about 2-2.5 liters in 16 hours while awake is adequate. If symptoms don't improve or you feel sicker tomorrow then you need to be treated. If you feel better, good fluid intake for another day then you should be fine. 1 episode of severe urinary tract infection is irrelevant to the current condition.

If mild it can. Having urosepsis in the past would make you prone if you had any other conditions like Diabetes, stones or urinary tract abnormality, immune deficiency etc- not other wise. Pl try not to self diagnose. Drink plenty of water, dont hold urine too long when you have to go. Keep the area clean and you will be fine.