It is true that for a sin to be mortal three conditions must be met: 1) it must be a grave (serious) matter, 2) you must be aware that it is a grave matter and 3) you consent to do so of your own free will.
I would not necessarily agree that it is difficult. I don’t think that just because we manage to rationalize out a way to argue something might be ok or only a small thing, that we are necessarily unaware of what we are doing. In fact, If we did not know something was serious, would we bothering engaging in such thought processes to make it seem ok? I wouldn’t. After all, it’s a lot of work. However, I can look back in my life and see plenty of times where I rationalized out excuses for myself and why this obvious rule shouldn’t apply to me in these circumstances before doing something that I clearly understood was wrong.
I think my wording here is bad-- but I am struggling to come up with a good way to put all this. However, C.S. Lewis does it very nicely in his opening chapters of Mere Christianity. I highly recommend reading this book–he [Lewis] does an amazing job of showing the existence of an absolute moral law, our inherent understanding of that law, our general failure to follow it and to make excuses for ourselves, the existence of God, and belief in Christian doctrine all with argument of reason. And that’s just the first five chapters. It isn’t even difficult to read or understand.