Your objection strikes my ears like that of a person who does not want to be "beaten down" into going to Grandma's on Thanksgiving. Really? Nobody's forcing you. It's a love thing.
I was a Catholic from birth to 40+ years. It was a mortal sin to eat meat on Friday (until the change in the ‘60’s) or during lent, or to not fast on certain days (Ash Wednesday, Good Friday). You are absolutely correct that there should be a willing heart, but most Catholics do it out of the fear that they will go to Purgatory (or worse) if they don’t “follow the rules.” My parents still don’t know that they are assured Salvation. Christ died for us to be free from constant worry of whether or not we have done enough good works.