He's blaming himself. There's really no other way to understand "awful" and "terrible" here other than in the sense of denigrating his own behaviour. This is confirmed in the next line beyond your quote:
So when I told old Spencer I had to go to the gym and get my equipment and stuff, that was a sheer lie. I don't even keep my goddam equipment in the gym.
There'd be no need for him to describe his gym kit as "goddam" equipment unless he wanted to emphasise the ridiculousness of his lie. At the end of the preceding chapter, we see that he's concocted his lie because he doesn't like having to talk to Spencer and he wants to get away:
I felt sorry as hell for him, all of a sudden. But I just couldn't hang around there any longer, the way we were on opposite sides of the pole, and the way he kept missing the bed whenever he chucked something at it, and his sad old bathrobe with his chest showing, and that grippy smell of Vicks Nose Drops all over the place.
The potential confusion in the passage you quoted comes from the word "terrific" because this can be used to mean something amazing, as in "I'm the most amazing liar you ever saw in your life":
terrific adjective (VERY GOOD)
a terrific opportunity
You look terrific!
However, it can also mean something in great quantity, and this is the sense it's meant in the quoted paragraph, as in "I'm the most prolific liar you ever saw in your life":
terrific adjective (VERY GREAT)
used to emphasize the great amount or degree of something:
The police car drove past at a terrific speed.