I’d argue that every sin committed is a sin committed out of weakness. Whether that is a weakness of will, a weakness of knowledge, or a weakness in caring about God and his will, it is still born of weakness. Every mortal sin would be a sin of weakness because in order for it to be mortal we have to understand that it is gave, and then freely chose to engage in it. This means that when we commit a mortal sin either our wills are weak, our dedication to God is weak, or both are weak.
I would also argue that no virtue is inherently easy. Certain virtues may be easier for some people than others, but that doesn’t mean the virtue is “easy,” it just means that you’re more naturally disposed to that virtue than another. For instance let’s say that I’m pretty good at being Charitable, I find it easy to give my time and money to others. This, however, is offset by that fact that I am horrendously impatient. The instances in which I am charitable do not become less efficacious because I find them easy to do, just as my acts of patience are not more efficacious because they are difficult. Performing an act of patience is -better- for me, but only because it’s helping me develop the virtue so that it will be easier to do in the future, not because it’s worth more due to difficulty.