Should I be coding this myself?

Question

As I've always liked doing things myself, I chose to code in C++ rather than Visual Basic at university because I could see what was going on and how it worked.

However, sometimes I feel like I'm wasting my time though, as there are so many 3rd party libraries out there.

Here's an example of what I mean.

A while ago I created my own collision detection function based on the x,y coordinates of the mouse and the dimensions of the sprite looping through each line checking for visibility of pixels. About a fortnight later I found out that there is already an identical function built into DirectX.

Doing it your self helps you understand how it does it but you cant possibly understand everything about programming, there's to much and it keeps changing.

Being young and relatively new to the commercial world I was wondering what experienced professionals opinion was on when to code something yourself and when to just used what's out there?