If you think you’ve done something so horrible that God can’t forgive you, you should realize that what you’re really saying is that your sin is more powerful than God. And that’s a lie! Nothing is more powerful than God – nothing. There’s only one sin that is unforgivable, and that’s blasphemy of the Holy Spirit (Mark 3:28-29). That involves speaking against the Holy Spirit, or saying that a work of the Holy Spirit is in fact from the devil. And from what I’ve been taught, it’s unforgivable because the person who commits it is so hardened against God that they won’t ask forgiveness.
If you’re concerned that you’ve committed this sin, it’s a pretty good bet that you haven’t, because your heart is still concerned about sin.
So, if you’re thinking you’re too far gone to be any good to God, you need to have a sit-down talk with Him, and let Him remind you how much He loves you. He’ll probably point out some people in the Bible who messed up pretty badly and still came back into a right relationship with Him.
It’s a good thing to be concerned about how you’re living, and whether you’re living for Him – but remember, you can’t be good enough to earn your way into His grace, and you can’t spend enough money to buy your way into His grace – but that’s what makes it grace- because it’s freely given to each of us; it’s His gift to us.