If I miss Mass because I went out of town but attended a different church, have I committed a mortal sin and do I need to go to Confession?
If you are a Catholic and couldn't get to Mass because you were out of town, you could still fulfill your Sunday obligation by attending another Catholic parish in the town or a nearby town you were visiting.
Attending either a Sunday Mass or a Saturday vigil Mass at the town or nearby town you were visiting would count.
Obviously, you could not fulfill your Sunday obligation by attending a Protestant denomination service because what you believe as a Catholic is not what they believe as Protestants. Protestants do not have a Common Union with Catholics on faith issues.
If you did attend a Protestant denomination service, this should be brought up the next time you go to Confession.
Note that I said brought up and not confessed as you probably had good intentions.
If you willfully did not go to Mass on Sunday, though you could have, it is a mortal sin and you have to go to Confession before receiving Holy Communion.
Important: Under these circumstances, (needing to go to Confession before receiving Communion), you can and should, still go to Mass but you have to refrain from receiving Communion until you get to Confession. We are a Church full of people with good-intended hearts, but that doesn't mean we should avoid our intellectual ability to discern serious sin from sin that is not serious.
You may ask:
- Why should I go to Mass, when I can't receive Holy Communion?
Answer: Because of the many other graces you receive at the high point of the Mass: the consecration of the Sacred Bread and Sacred Wine into the Body and Body of Jesus.
This consecration itself brings down its own blessings on us for the following week. Those who blow off Sunday Mass because they privately claim, they can talk to God privately without going to a Church, are giving themselves self-inflicted spiritual wounds which will lead to a lack of ability to make good decisions and choices for that and future weeks to come.
There have been times in the past where I have had to refrain from receiving the Blessed Sacrament because I haven't been able to get to Confession. When this happens, I [optionally] say a Rosary in the pew while others are receiving and make a silent Spiritual Communion.
O Lord Jesus, I believe that You are present in the Most Blessed Sacrament of the Altar.
I love you [above all things]|[with all my mind and my heart] and I long for thee in my soul.
Since I cannot receive You now sacramentally, at least come spiritually into my heart.
I embrace myself entirely to You and unite myself wholly to You.
Never permit me to be separated from You.
Come Lord Jesus [optional add-on:] and glorify yourself through my weak body.
I hope this helps,