Even if you have failed God in the past, if you repent and forsake your old ways, God can still use you mightily. (He specialises in using the most unlikely people in powerful measures.)
If you are backslidden, the Lord can still restore you if you will allow Him to. Go back to Him in prayers, start studying your Bible, and begin to walk with Him in obedience in fellowship with other Christians.
As you take one step towards Him, He will take 99 steps towards you.
MEMORISE: I will heal their backsliding, I will love them freely: for mine anger is turned away from him. (Hosea 14:4)