Hi, I do not think the church considers it adultery if you get married outside of it. For the sin to be adultery, you or your boyfriend would have to have been first married to another person who is still alive. Right now if neither of you have been married before, I think the word to use would be fornication— sex outside of marriage.
It may feel uncomfortable to go to a priest to ask for advice, but if you are thinking of wanting to return to the church, I would guess that you have been to church. A priest can answer all your questions.
My advice would be go to a recently ordained priest, I have found them to be very gentle in the confessional. Although you may want to schedule an appointment rather than just showing up on a Saturday afternoon before Mass, so that you would have enough time to discuss everything.
I’m praying for you and your boyfriend.