If you don't mind my asking, what have you done to make you feel like have failed in living up to "God's will?" It's impossible for any of us to live up to God's will. That's why Jesus came. OK? We have grace!
Period.
There is nothing you can do yourself to make God love you less or more either way. He already does, and its not changing because of anything you say or do, good or bad
Think of the thing you love most and He loves you millions of times more. I'm dead serious.
I think what you can't live up to is your own ingrained legalism. You have not failed God. He loves you, no questions asked. Just accept the fact that you are accepted by God. Accept his total, unrestrained forgiveness and learn to FORGIVE YOURSELF. Just sit back, take a deep breath, and accept the fact that you are accepted. Jesus died to be with you. He's not going to turn his back on you now because you slipped up doing something.
I've never read the Purpose Driven Life (it's probably great) but I know a book that might fit your situation better, in my humble opinion. If you don't mind giving it a chance, it might change your life. It's called the Ragamuffin Gospel by Brennan Manning. I can even buy one and figure out some way to send it to you if you would like.
You are forgiven. You don't have to do ANYTHING to make God love you. Say, believe it, and hold on to it.
I believe your desire to follow Christ is TOTALLY there. You just need to take a different approach in your perspective on life. God loves you AS YOU ARE. You do not have to work like a dog, be perfect, and feel guilty all the time to make Him love you. He loves you right now. For me, that is much better motivation to get better . . . love rather than fear.
One more thing . . . you believe God called you to follow Him at some point in your life, right? Well, then, when He did that, He KNEW what would happen in your life in the future. He KNEW whatever mistakes you would make. And He called you anyway!
Praise Him! He knew that you would pull through your struggles. You are 15, He has so much for you. Your whole life is in front of you. I'm 18 and SOOOOOOOOO much has happened to me in the just last 3 years. And I really wasn't even a Christian then. There is hope. There are great things in your future.