As'salaam Alaykum, Sis...
I am very sorry to hear of your grief, but I can certainly understand.
First, I'd like to advise... NEVER abandon ALLAH and ISLAM for anyone or anything!
You were once a Christian, as you stated... and you abandoned that religion because of your heart and beliefs; which did not coincide with the religion itself. ALLAH led you to ISLAM, because of this correct thinking and believing... in the ONENESS of GOD (ALLAH); which IS ISLAM. You were a Muslim even while practicing Christianity, because you had already embraced the Message, the Way... the Path... straight to ALLAH!
I can not advise you in any way on the issue of divorce... only ALLAH knows best! Therefore, you must consult ALLAH's law concerning this matter... the Qur'an... and choose according to what will leave you in good standing with ALLAH.
We are to move away from anyone or anything that will come between us and ALLAH... He knows best!
Follow what is right... even if it goes against your FEELINGS, which are obviously VERY strong for your husband. But never abandon ISLAM because of your grief of a relationship with someone other than ALLAH.
ISLAM also means PEACE... and when you are in submission to ALLAH (ISLAM)... you are accepting PEACE and ALLAH for yourself. What is better than this?
Be the best Muslim that you can be for ALLAH... this should help you with your decision.
Peace and blessings to you... and may ALLAH guide you in the correct way and have mercy in light of your circumstances.
The Meek One, A Lady angel