I'm a Muslim living in the United States. I have committed a sin that I can't forgive myself for till this day. I met an American girl, that I fell in love with and ended up marrying her. However, during the time we spent before marriage, she convinced me to go to church with her and get baptized at her church. I know deeply that this was Kufr and associating Jesus to Allah, but I went ahead and did, and may Allah forgive. Now I came back to Allah and practice Islam, to the best of my small knowledge. My question, in the matter, will Allah ever forgive me for associating someone to Him and performing this hideous ceremony at the church?

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

There are four rules concerning entering a church.
First, when a Muslim in dire necessity enters a church to pray in it, this is lawful as reported from Hasan al-Basri , Caliph Omar Ibn Abd al-Aziz and al-Shaabi , …etc.
Second, when a Muslim enters a church to see what is inside it, this is also lawful as it was reported from Ali Ibn Abi Talib that he entered a church and looked around at pictures. It is also reported that Omar made a contract with Zimmi (non-Muslims living in Islamic land) whereby they would widen the entrances of their churches to allow Muslims and passers-by to go past it or through it.
Third, when a Muslim enters a church on one of the disbelievers' feasts to sympathize with them or congratulate them, this is strictly forbidden al-Baihaqi reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "Do not enter the churches of disbelievers on their feast days as the Anger of Allah befalls them" .
Four, when the Muslim enters a church and perform the rites of its people, this is pure Kufr disbelief. The one who has done this, should repent to Allah and multiply Istighfar, and whoever repents Allah forgives him and it is unlawful for such a person to despair as he actually becomes a beloved of Allah when he repents from previous sins.
Allah knows best.