Who told you a Mortal Sin does not exist? The tendency in the East is not make a hard and fast formal distinction between mortal and venial sins. Certainly there are greater and lesser sins. Your conscience should be a big help to your here, if the Law of God is written on your heart.
Ultimately, only God can judge the severity of your missteps, assuming that you have tripped up now and again

, I know I have.
One should just go to confession at vespers before Divine Liturgy, every time if necessary. Confession is good medicine, especially if the priest knows us and can discern trends in our choices and behavior. We can work on those things and better ourselves, that’s what a spiritual director is for.
There have been Catechisms produced by Eastern Catholics for Eastern Catholics. I don’t know about the Maronites and Chaldeans but the Byzantines have produced a simple one in three short volumes (see Light for Life
here and
here). The UGCC is ready to release a new one, if they have not done so already, I have no info on it.
An older catechism by Father Casimir Kucharik (a UGCC priest) called “Our Faith” might be had in used book shops or on Ebay. Don’t look for the publisher Alleluia Press to have any because I got the very last one a few years ago

.
The Divine Liturgy serves as a catechism, and in the Byzantine tradition (not sure of the others) it really works. One must follow the entire liturgical cycle but practically everything Byzantines believe is discernible in the Divine Liturgy and the Divine Praises.
Michael