If a text is too difficult, work through an easier text on the same topic (or prerequisite topics) first. For calculus, as preliminary training, I recommend Thompson's (original, 1914) "Calculus Made Easy" (basic intuition; you likely already have that if you're learning multivariable) and Courant's "What Is Mathematics" (partially as an introduction to proofs and various topics that won't be brought together in this manner in standard introductory courses). Spivak's text is great as a first exercise in meticulous proofs, but do take an actual analysis course after multivariable calculus (the current best book for this role seems to be Pugh's "Real Mathematical Analysis", which works as an improvement over baby Rudin). For multivariable calculus, I've heard good things about Hubbard's "Vector Calculus".