No doubt you were sincere when you made this promise to God–but He knows if it wasn't possible for you to carry through on it, and He doesn't hold it against you.
Only one sin is unforgivable–and that is the sin of turning our backs on God and refusing to heed His call to repent and believe in Jesus as our Savior and Lord. When we do this, we are calling God a liar, for we are refusing to believe the Holy Spirit's testimony that Jesus was the divine Son of God who died for our sins and rose again for our salvation. This is why the Bible says that "whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin" (Mark 3:29).
Don't let the memory of this well-intentioned promise keep you from Christ. Satan would have you believe that God won't forgive you–but the Bible says otherwise. The Bible says, "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness" (1 John 1:9). By faith ask Christ to come into your life and cleanse you from sin, and commit your life without reserve to follow Him.
We may fail sometimes to keep our promises to God–but He will never fail to keep His promises to us. He loves us, and when we commit our lives to Christ we know that "no one can snatch them out of my Father's hand" (John 10:29).