Will Allah forgive the sins of a person who deliberately abandoned prayer and fasting in his youth, then repented and became committed, and is now fearful of Allah's punishment?

Your feeling that Allah will not forgive you is a whisper from Satan, for Allah is Forgiving and Merciful, and He forgives all sins for whoever repents. The one who repents from a sin is like one who has no sin. Think good of your Lord and turn to His obedience. Make up for the prayers and fasting you missed. You are upon great good by observing the obligatory duties and the supererogatory acts.