Continuing on from Bushum
You would be marrying an already married man thus committing adultery. You would be in a state of mortal sin and not able to take the Eucharist. You could go to confession and get absolved but you would be immediately continuing in the same sin.
As far as I know, if you went to confession as outlined by Bushum, as an annulment would not have been granted before marrying, and
assuming you two continue to live as husband and wife, that confession would not be valid
if there is no determination to live chastely until an annulment is/may be granted.
So if there is not any purpose of amendment then absolution cannot be granted as the person does not intend to change their ways.
(assuming you are still having marital relations) , so then nothing would be forgiven. You would have to confess not only marrying outside the Catholic Church, but also any sexual activity between you both - as these are adultery until an annulment is obtained.
The question of a sacrilegious confession due to the fact
“if” there is no firm purpose of amendment (living chastely or better yet separately, until an annulment is obtained) arises in my mind too. And a sacrilegious confession is a mortal sin.
By going ahead and marrying outside of the Catholic Church without a dispensation from the Bishop, (and in your situation I highly doubt it would be granted), is also a mortal sin as far as I know.
Also committing a sin with the idea - I can do xyz and then go to confession because “God will forgive me” is the sin of presumption, which is another mortal sin.
Try to put feelings aside - and will the good of both your eternal souls and obey God and the Church. If he truly loves you, respects you, then surely he would be prepared to obtain an annulment
before getting married in the Catholic Church.
What would you do if an annulment is not granted? Your choice is to separate and remain chaste, or continue to commit adultery? Please don’t put the happiness of this life above the happiness of
eternal life.