OP, your aspiration is possible but difficult. CFD is a rarefied field of heads-down, highly quantitative minds. If you were outstanding at math in HS then I say go for it. Going back to undergrad isn't a bad idea. You'd be a non-traditional student, but I knew a few of those when I was in undergrad who did well. You should be pretty comfortable in math through vector calculus and linear algebra; if you're really struggling, you might want to reconsider paths. You have to be ready for 3-4 years of hardcore aerospace engineering and math courses, which could be a tough transition. Good luck.