Dear friend, on the point of a grave sin, in my mind, every sin is grave if forgiveness is not asked for.
As far as visit to Church for Divine Lithurgy (Theia Leiturgia) is concerned I think that reason or reasons for not going should be considered.
On the point of calling Divine Lithurgy Mass, hey... Ut Unum Sint.
Orthodoxia i thanatos.
Our Most Blessed and Pure Theotokos and all the Saints pray to God for us, and firstly me, a sinner.
Holy God, Holy Mighty, Holy Immortal, have mercy on us.
....and in ONE, HOLY, CATHOLIC and APOSTOLIC CHURCH....