Thanks for your question. God's Mercy is superabundant, but at what a cost for our salvation: the cruel death of His Only Begotten Son. So, we owe Him everything, our lives, our possessions, and especially our time. He gave us a commandment to keep Holy the Sabbath so going to Mass every week is a divine command.
When someone deliberately disobeys God, especially when it is a direct command, that is a grave matter. That is why the Church calls it a mortal sin, because it damages the relationship on so many levels and deprives the soul of sanctifying grace, an essential condition of our justification. If it were marriage, if would be like your spouse saying he can't keep that special date with you because he has other plans — like playing golf with his buddies. How about if, instead, he was with another woman? That's more like how God sees it. He is planning an eternity with us and He needs us to be enlisted in the relationship.
The Bible is full of admonitions that we must remain faithful and obedient to have a covenant with God, knowing we trust in His Mercy when we do fail. The Protestant notion that once saved always saved doesn't account for a person walking away from the relationship, and there isn't a Protestant church in existence that hasn't seen many who once professing the faith, simply walked away. It happens everyday, in every church.
- love of God, and
- love of neighbor
These are the ingredients in the soup you want to be consuming. A daily walk of faith, fueled by the grace of God, who literally gives us the strength to do all these things, that's how we get to Heaven.
If you reread the whole New Testament you will see the need for perseverance everywhere, so I won't list all the references, but hope you will read it anew. At the same time, God doesn't want us to suffer discouragement when the going is difficult, so there are many assurances that He is with us and is leading us along the right path. He carries us when we have no strength.
This is the deal, grace acting on nature. God wants us to participate in this effort because that is part of the glory of a Father: to see his children striving and achieving the prize; children empowered by a loving Father to do what He asks. That is His and our glory.
Lastly, the most powerful means of divine grace come to us through the Sacraments, all given as gifts to seal this Covenant. They will power you through even the worst of times. I'm sure Saints Peter and Paul, Mary, St Joseph and all the other saints wished there wasn't hardship and suffering in following the Lord, but they picked up their crosses and went on despite the cost.
Christianity may not be easy but it's worth it to know and love a God who has revealed himself as a simple baby in a manger.