You may confess to any priest in whatever parish. As a priest who actually hears confessions, I can tell you that I rarely remember anything from confessions, and I don’t even think about whether or not someone has been really forgiven–I assume they are, given that they came in, confessed, and I said the words of absolution.
You’re overthinking this. If you intentionally omitted a mortal sin, you must reconfess what you confessed in that confession, as well as the sin you omitted, as well as the omission itself.
If you don’t know if you intentionally omitted it, or didn’t know you weren’t supposed to do that, by definition, you haven’t committed a mortal sin in doing that, since you wouldn’t have had full knowledge. If you just forgot to mention it, you’re also in the clear–just mention it at your next confession, and reconfession of everything else is not necessary.
Be very careful about getting worked up about these kinds of things. It’s very tempting to fall into a lack of trust in ourselves in how we approach the sacraments and whether or not the sacraments work. For the sake of your peace of mind, examine your conscience, confess courageously, and trust in the mercy of God.