Can i recover my health?

ok, basically for 3 years now my eating habits have flew out of the roof. on a daily basis i ate 12 packs of crisps 2 litre coke and 10 big cookies plus dinner, lunch, pudding..... etc.... everyday so its a big deal... im very thin.. actually i found out my BMI is danger zone underweight.. no idea why... but because everyone tells me "your gonna die eating like that" not fact.. just opinion and for 3 weeks i have quit all junk completly with 1 pack of shortbread a week.... can i really reduce my high cholestrol? lets be honest i proberly have horrid atteries by now. im trying excersise too... but is it worth my time? can ireally make results... im in my teens.