I am currently self-studying Calculus by M. Spivak and before I started I was well aware of the fact that it is not an easy read. Right now I am going through the problems at the end of the very first chapter and I would like a piece of advice. There is a somewhat substantial part of the problems which I am not able to do myself without getting hints from the book of solutions and I would like to hear from somebody who has been through it whether I should persevere and keep going or whether I should put it aside until I gain some more mathematical maturity. Even though I havn't been able to do all of the problems I feel as if I have gained a lot just by trying hard and then try to understand the given solution. I've heard that it will get easier along the way - not because the difficulty of the problems gets lowered but because one gets used to the style of the book.
Cheers
When I started in college my calculus professor used Spivak's book as the textbook for the course, I remember that was really hard at the beggining and mostly of the problems I couldn't solve by myself, but at the end of the course I was solving the problems by myself, and I felt that I improved a lot just for trying hard with Spivak's book. So I think you can give a chance, and try a little long, you can use other books to help to, like Apostol's book, to see things that wasn't clear from another point of view.
Well, I hope that I've helped you.