'Throwing away' three years does sound harsh, but the quality of those three years is what makes the decision and not the time, of course. It's always been my philosophy that it's not anyone's job to stay with someone if they're not happy with them, just to keep the other person satisfied. A happy relationship takes two!
Since he's just your boyfriend, and your current boyfriend is not the energetic person you said he used to be, it's only natural to move on. Perhaps this might also be a wake up call for him as well, who knows.
I say look into the second chance!