I've had a very similar experience, though the completely opposite situation from you at the moment! I adored maths and physics, and really wanted to become an astronomer. But no matter how hard I tried, physics and maths were just too difficult for me so in the end I am studying graphic design since design is something that I can do. But here I am now, currently creating work for the likes of ESA, ESO, and NASA/Hubble, as well as freelancing for other astronomy societies/organisations here and there. Who said dreams don't come true, eh? (: (even if I am only doing menial tasks as an intern... lol)
I'd personally say your situation is ideal because engineering has a very vast network of career opportunities for you, even ones you've probably never thought of/considered. You can still go into illustration if you have a good portfolio, and you can always go back into engineering as a back up if illustration doesn't work out. Good luck! (: