BBT as birth control what do you think?

Hey there, 
​I currently have an IUD (Mirena) but with lots of side effects, so it's definitely gotta go. 
​Problem is, the pill/hormones in general don't work out for me, so the only thing left is bbt/cm tracking and counting days on the calendar (I have pretty regular cycles) 
​I'm 18 and in university, so I just want to know if there is someone here who has experience with it? Something I really need to know? 
​Thanks in advance!