I'm 24 and unmarried, I masturbated for over 14 years. I recently learned the harms and that it's a very big sin. I stopped and I have been praying Allah to help me keep away from such actions, but will I ever be forgiven? I also fear what will happen after marriage. Will I be normal? I intend to get closer to Allah and, al-Hamdulillah, I am doing good deeds (praying, fasting, reading Qur'an, staying away from Fawahesh,...) I know I was wrong but I am trying to make up for it. What do you have to say to me? What else should I do? What about sexual intercourse after marriage, will it be affected?

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

We ask from Allah steadfastness and forgiveness for both you and us. We advise you to continue doing good deeds even if they are small or few. The best actions are those done perpetually even though they may be small in size or quantity.
We advise you to be in the company of righteous brothers and to avoid the company of evil persons. Also we advise you to seek refuge of Allah, multiply your good deeds and continue asking Allah to grant you steadfastness, forgiveness and guidance.
As for your question, 'if you will be forgiven', Allah Says (interpretation of meaning): {And verily, I am indeed Forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them, (till his death).}[20:82] 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] Allah also 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].
As for bad effects of masturbation, you may consult medical professionals.
Finally, we ask Allah to keep you away from the bad consequences of that practice and grant you a happy marital life.
Allah knows best.