I believed I had committed the unpardonable sin at one time. Worst feeling ever--I felt like I was falling into a black hole, and I took my feelings as a confirmation that God had let me go and abandoned me forever. I thought I had rejected Him one time too many, and that my heart had become too hardened to be healed.
But over time, after a lot of struggles and prayer and study and people encouraging me, I realized that my feelings weren't what I should rely on. I realized I need to believe Jesus' words when He says, "He who comes to me I will in no wise cast out." John 6:37.
I realized that my fears and worries about the unpardonable sin were evidence that God was still working on my heart, and that He was calling me back to Him. God doesn't care how sinful you are, or how low you've sunk; if you're willing to come, He's willing to forgive and heal you. Read the parable of the father and prodigal son in Luke 15. Picture the father running to greet his son and put a robe on him. That's what God wants to do for anyone and everyone, "whosoever will" (Rev. 22:17), if they're willing. Remember that God is FOR you, not against you.
"If God be for us, who can be against us? He that spared not His own Son, but delivered Him up for us all, how shall He not with Him also freely give us all things?" (Rom. 8:31-32).
And if you're worried you haven't repented properly, remember that even repentance is a gift from God. (Acts. 5:31, 2 Tim. 2:25). It is the goodness of God that leads us to repentance. (Rom. 2:4). Satan likes to discourage us and think our cases are too hopeless for God to work with, but God is willing to work with anyone, even Paul who persecuted the Christians, and King Manasseh who did multiple horrible things (2 Chron. 33). They were both saved because they responded to God's call to repentance, and trusted in God's mercy. Jesus can heal the hardest heart, if that person is willing to let His light melt them (John 3: 21, 1 John 1:7).
Don't listen to Satan's lies that you can never be forgiven. In 1 John it says, "If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness" (verse 9). Jesus has the power to cleanse us from all unrighteousness if we're willing to turn away from evil and accept His righteousness. Believe it. Even if you don't feel forgiven, believe God's words, because He can't lie. He'll heal your backslidings and love you freely (Hosea 14:4). He'll take your heart of stone away and give you a heart of flesh (Ezekiel 11:19). He'll create in you a new heart, and renew a right spirit within you (Psalm 51:10).
I know, because He did it for me. <3