Funny that you're asking the question because a few years ago when I was in high school I was in a similar situation and incidentally started off with Courant and John's Introduction to Calculus and Analysis. I must say, I greatly benefited from the book and didn't find it too difficult at all. It is very well written and introduces many concepts quite well. However, the book I used (which is, I think, not exactly the same as the one you're referring to, although similar) is very thick and it can be a bit frustrating trying to work through all of it as a beginner. It goes into great depth and in your situation it may be a good idea to go for a text that introduces a variety of concepts in analysis, but is a bit thinner and easier to work through. I've found "A first course in real analysis" by Protter and Morrey very good in that respect, as it introduces analysis in one and several dimensions, metric spaces, infinite series and many other concepts and gives a good overview. The explanations are very clear, the proofs make good sense and the exercises are very doable (Courant's exercises can be a bit more difficult at times, which is good, but in the beginning a more leisurely going speed may make it more fun). However, you can't really go wrong with Courant either, in my opinion, so whichever you choose it should be very doable.