If I let God down, will he still love me?

It sounds as if you are feeling quite down - almost as if even God has forsaken you and there is no hope.  If this is so, it's important that you find someone to talk to - a trusted friend, family member or a counsellor - to help you to gain a better understanding of what you are experiencing.  As far as the idea that God offers something to you then turns his back forever if you don't accept it - there is absolutely nothing in scripture that will back that up.

God isn't like some of the people in our lives that abandon us when the going gets tough - in fact the bible says he will never leave you or forsake you (Deuteronomy 31:6 and 31:8).  He's not fickle or a fair weather friend - his love lasts forever (Isaiah 54:10, Psalm 136).  God is kind - Titus 3:5,6 tells us it is his kindness that saves us, not anything we have done to earn salvation.  He is faithful - in fact, even when we are unfaithful to him, he remains faithful to us (2 Timothy 2:13).

And what about his mercy?  And his grace?  There's plenty of proof in the bible that God is there waiting for us when we are away from him.  Nothing illustrates the father's love, mercy, grace, and faithfulness better than the story of the prodigal son (Luke 15:11-32).  This is really a story about our father God and his passionate love for us, his children.  Finally, God is also faithful to forgive us (1 John 1:9).  Even when we've painted ourselves into the tightest corner, he offers us a way out (1 Corinthians 10:13).

I hope these scriptures encourage you and give you fresh hope.  And as I said in the beginning, make sure you talk to someone about the way you're feeling.