What you are experiencing, that is, the weigh of the past sins, is a sign of the presence of the Holy Spirit: without grace, it would be impossible for us to experience repentance and to seek reconciliation with God. Thus indeed you are blessed. Allow me to quote the Catechism:
1451 Among the penitent's acts contrition occupies first place. Contrition is "sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again."
1452 When it arises from a love by which God is loved above all else, contrition is called "perfect" (contrition of charity). Such contrition remits venial sins; it also obtains forgiveness of mortal sins if it includes the firm resolution to have recourse to sacramental confession as soon as possible.
Regarding the Sacrament of Reconciliation, remember that it should not be seen as an encounter between a guilty servant and an angry master, but as a moment when heaven and earth are united and Christ Himself is present in His minister, who is said to operate in persona Christi.
To Christ you confess because you long to be reconciled with Him, that is, you want to be His friend, just like the good Peter jumped from the boat and crying told the Lord: "good Lord, you know everything, you know that I love you!". And it is a fact that between two people, the one who loves the most rejoices the most at reconciliation...and Christ loves you of an eternal and infinite love...thus when you confess, no matter how sorrowful you feel, no matter what sins you confess, remember that Christ rejoices at your presence and at your repentance.
Talk to Him with an open heart, naturally, without fear or constraints. Try to mention numbers, if you remember, but this may be hard sometimes, and thus you simply say it, most naturally. Mention kind, that is, be specific, have no shame, for God sees in the secret, and would be displeased that you refrain from confessing what he already knows and longs to forgive as long as you ask for its forgiveness.
And in the end, you may add that you seek forgiveness for these and for all sins which you may have forgotten, thus acknowledging humbly your own limitation in doing a good examination of conscience.
Of course, you will learn more in RCIA, but this is what I wanted to share with you.
At the beginning of the Church, the Sacrament of Reconciliation was much more strict. Sinners often had to enter an order of penitents and live entire years in penance and mortification, and often they were allowed to confess only once in a lifetime, which often occurred on deathbed. Throughout the centuries, not just confession, but even spiritual direction has become much more open and available, and this is the sign of God's ineffable mercy. So rejoice in a blessing that many in the past longed to have with their whole heart, and make use of it frequently, every 15 days or every month, even if you are only conscious of venial sins, for God wants to do something great with you: it is His will that, by persevering, you shall be made perfect in the fullness of time.
I recommend the reading of Story of a Soul, by the young Doctor of the Church Therese of Lisieux, as well as the Introduction to the devout life of St. Francis of Sales.