I am 24 years old. I do my best to be a good Muslim. But I have something that is ruining my life. When I was in college I fell in love with my colleague. He was a good man; he told me that he is not ready to marry me. Because I needed nothing but marriage we left each other. I married another man and he is so good and he does his best to make me happy. The problem that I can't love him at all I always remember my first love; I can't help it. I swear to Allah I can't love my husband, but I treat him so good I keep all his rights but the only thing is my heart is not mine. I love Allah and I don't want to make sins. I don't want Allah to keep me away from His Mercy. I don't know what to do. Will Allah judge me with what my heart feels? Am I doing a great Ma'asiya...? Please answer me. Is it a sin if I asked for divorce (Talaq) only because I don't love my husband?

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.

Dear sister, know that asking divorce due to not feeling affection to one's husband is a mistake. Not being in love with someone is not a sound excuse to ask for divorce. If you do so, we fear that you will lose the Paradise. The Prophet (Sallallahu Alaihi wa Sallam) said: "If any woman asks her husband for divorce without any strong reason, the odour of Paradise will be forbidden for her" [ Ahmad , Abu Dawood and Ibn Majah , Sheikh al-Albani considered it sound].
Know that many houses are not established on affection as Umar Bin al-Khattab said: 'A few houses are established on love even though the people live together by Islam and by ancestry' .
The affection you have been feeling in your heart "first love" as you mentioned, will not continue for a long time. After a while surely it will vanish since all human beings tend to forget. So, you are obliged to forget your past. If any such fantasies bother you, you should get absorbed in matters that make you busy and take your mind away from them. Being busy in remembrance of Allah is much useful.
On the other hand, you should show your love and affection to your husband and get close to him for the purpose of loving him only; you may assure him that you love him. The Prophet (Sallallahu Alaihi wa Sallam) permitted lying for this purpose. He said: "Lying is allowed only in three cases: falsehood spoken by a man to his wife to please her, falsehood in war, and falsehood to put things right between people" [Reported by Imams al-Bukhari and Muslim ].
However, if the lack of love for your husband is limited only to your heart then you will no longer be considered a sinner.
Allah knows best.