I'm 35 years old and I did not perform prayers regularly but now I started performing the prayers. The truth is that I'm so scared that I will not be forgiven for the past years of disobedience. So, I'm scared I will be in hell forever? What can I do?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Allah loves the repentant, accepts their repentance and forgives their sins no matter how great they are. Allah Says (interpretation of meaning): {…Truly, Allâh loves those who turn unto Him in repentance and loves those who purify themselves (by taking a bath and cleaning and washing thoroughly their private parts, bodies, for their prayers, etc.).}[2:222]. Allah also Says (interpretation of meaning): {Say: "O 'Ibâdî (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allâh, verily Allâh forgives all sins. Truly, He is Oft-Forgiving, Most Merciful.}[39:53].
Besides forgiving their sins Allah also changes their sins into good deeds.
Allah Says (interpretation of meaning): {And those who invoke not any other ilâh (god) along with Allâh, nor kill such life as Allâh has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace; Except those who repent and believe (in Islâmic Monotheism), and do righteous deeds, for those, Allâh will change their sins into good deeds, and Allâh is Oft-Forgiving, Most Merciful.}[25:68-70].
Know that the day you turn towards Allah repentant that is the best day in your life. As the Prophet (Sallallahu Alaihi wa Sallam) informed Ka'b Bin Malik when Allah had accepted his repentance. The Prophet (Sallallahu Alaihi wa Sallam) said: "Be happy with the best day that you have ever got since your mother delivered you". [Reported by Imams Bukhari and Muslim]
Allah knows best.