Is missing Mass occasionally really a mortal sin? I have heard it is and can not understand why? To never go, yes. But to miss here and there, I don’t have a problem with it.
Lisa,
I’m not going to focus on what does/doesn’t put your soul in mortal peril, because in the end, only yourself and God know your heart and the state of your soul, regardless of what the rest of us see you doing on the outside.
I hope my response can help you understand the importance of attending mass.
When you find yourself not really in the mood to go to mass, or “too tired” to go to mass, think of it as being a sacrifice - “God, I am tired and I really don’t feel like going to mass today, but I will go anyway as a symbol of my dedication and love for you.”
Mass is 1 hour a week out of our lives that we dedicate to nothing/no one, but God. It may be tedious, it may be inconvenient at times. But during those times, we must find the will within us to go
anyway, and this will power is an action that demonstrates our commitment to the Lord… and I am sure this means a lot to Him.
We must also remember that sure, we can pray/think of God outside of mass, but we can ONLY receive HIM, Jesus Christ HIMSELF, in the flesh and blood,
during mass. Think about it - Jesus Christ is right there, physically… and we take him inside of us!

This is an amazing, incredible miracle that can happen only during mass!
So yes, going to Church is very very important. It is very significant to us because we receive HIM in the flesh and blood, and it is very significant to God because we show Him our devotion by sacrificing our time and being there even when we are tired.
I hope that helps,
God Bless.