All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
We believe that Allaah wanted good for you and hence instilled the love of being religious in your heart. As a result, you sought the truth in Christianity, and due to the sound fitrah that you had, it became clear to you that the Truth is in Islam as it is in harmony with the sound fitrah. Henceforth, we congratulate you for this blessing, and we ask Allaah to make and keep us and you firm upon the Truth, until we die, to admit us to Paradise, and to protect us from the Hellfire.
There is nothing wrong with the happiness that you felt because of reading the biography of the Prophets and the righteous and because of performing good deeds, Allaah willing. There is also nothing wrong with the beautiful feelings that you have when you remember those days, and you are not obliged to repel them. However, let your happiness with Islam be greater as it is the way that makes you achieve happiness in the worldly life and in the Hereafter. Allaah says (what means): {Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life, and We will surely give them their reward [in the Hereafter] according to the best of what they used to do.}[Quran 16:97]
Ibn Al-Qayyim said in Al-Jawaab Al-Kaafi, “The pious benefactors won the bliss of this worldly life and that of the Hereafter and they attained a pleasant life in both worlds. Having a good soul and the delight of the heart and its joy, its enjoyment, its happiness, its tranquility, its soundness, its light, its capacity and it being safe by abandoning forbidden desires and misconceptions, [all this, indeed] is the true bliss.”
Allaah Knows best.