For me, the leap to multivariate calculus gave me a lot of trouble.
Differential equations was doable but no longer fun for me, either.
It was a combination of multivariate calculus, linear algebra, and differential equations where I just wasn't having fun with math anymore. Those subjects represented the end of pure math education for me, and later engineering classes requiring knowledge in those were also not a ton of fun. Went from a self-described math and science guy to just...not describing myself in that way anymore.