Everyone failed to mention this, but you should take a class that instructs how to prove things. Or at the very least get a book about it. I also got Spivak when I started Calculus, and while the theorems were the same as what I studied in Calculus, I couldn't understand the proofs.
I took a proofs class and I'm now taking Real Analysis and doing well. I don't think I could have done well without that introductory proof class, and I think I'd do even better if I had taken a logic and set theory class, which leads me to my next point.
A lot of people are fine with seeing a few basic ideas (like the kind of stuff you see in an intro to proofs class), and run with them. I, for instance, am not one of those. I like understanding things thoroughly, so the more detailed classes (and perhaps maybe the more rigorous ones?) work better for me. However, like I said before, taking a class about how to write and understand proofs was enough for my real analysis.
Bottom line is, you should get some idea of how to prove things. If you're looking for books,
here's the book I used in my proofs class. I think it does a fantastic job explaining the ideas, but the problems aren't very challenging. They just help you understand how to set up a proof, but nothing more.
A really good book with good explanations is
this one, which can be bought for way cheaper at other websites.
This book was recommended to me before, but I never got it because it's too expensive.
Lastly,
this book is being recommended here a lot. I don't like the way it explains things, but it might work for you.
Also, what mathwonk said is really important. Reading mathematics is doing mathematics. You won't get a lot out of it if you don't sit and re
write the proofs as you go along.
Also, the problems in those books are hard; you might spend a few hours on a problem until you figure out how to crack it. It gets better as you keep advancing though. These problems are also different than problems you've done before. With computational classes you usually start manipulating something until you get the results, and a lot of the time you do it on ``auto-pilot". Here you have to think a lot before you start manipulating things.
One last thing (promise). Analysis isn't hard. It just requires a lot of time. So be prepared to that. Also, don't stress out too much if you don't get to do analysis until you actually take a course in analysis. Take the classes you need to take to get to analysis (including a proofs class), and be prepared to work hard.