As kids, we make our first confession before our first communion. Why is it different with adults?
CAF is not really a good place to get advice on this topic. Call your parish, they’ll have the right answer for you.
That said, here’s my speculation: if you are validly baptized, they will tell you that you can make a confession. Indeed, all Catholics are required to receive Holy Communion at least once a year-- and ideally, it’s at Easter! But if you are living in mortal sin precludes this. You will probably have to live as “brother and sister” until your marriage is accepted by the Church so as to not commit that particular sin. And mention to Father in confession that you are in process of convalidation, he’ll know what to do.
Good for you to desire confession! I’ve found that my confessions have gotten better and deeper the more often that I avail myself of the sacrament. I am not very good at examining my conscience-- and guides like the one posted above help, but are no substitute for practice.