All the answers in this thread thus far are only PARTIAL.
A complete answer would go along these lines: A past sin is between you and God as the priest well said, since your eternal guilt has been forgiven but temporal reparation may still be left to atone for.
The main concern at this point is both your and your bf’s suffering - that the past may be brought into the present. That is what needs to be addressed - here is where the answers in the thread seem partial.
What you and your BF are proposing&prospecting yourselves to is engagement, marriage and constituting a family. Your entire relationship should be conducted with the end goal in mind and an eye on the prize of eternal beatitude in heaven.
Trust is built on truth.
In the present you are addressing the issue of truthfulness and honesty while building a relationship. The keyword “while” places your relationship as an ongoing process.
In the present, you need to guarantee to your BF that the past won’t come back to endanger your relationship. Another keyword: WILL. That your will is to be with him and to work on your relationship and thus anything in the past is secondary and unimportant to your present construction.
Afterwards, “sins of the past” can indeed contribute to strengthen your trust, to increase the love between you. Exactly in the measure of your will: that you state your will not to commit those sins again or evermore, that they served only for your repentance and greater value you attribute to your present love, to repent. [And this is a direct consequence of "“go forth and sin no more” John 8:11.]
Again, an exaggerated honesty can ruin your relationship while it is about to flourish - and that is not wise. In the meanwhile, know that those “dead zones” in both your personal histories go against trust and should be addressed somewhere down the road when conditions and the relationship itself so permit.
[The above draws mostly from “Love and responsibility” by Karol Wojtyla.]