The suggestion about switching to the active voice ("X motivated me" rather than "I was motivated by X") is probably the more important point here. It makes for clearer, more direct writing. If nothing else, it saves a few characters and provides a little bit of sentence variety if you find every line beginning with "I".
If you keep it as is, then a comma.
And as several people have pointed out, yes, it's more properly called a dash than a hyphen. Even more super-properly, as I learned it back at Our Mother of Perpetual Obligation, we're really talking about an em-dash (or m-dash or long dash) because back in typewriter days, it was the same width as the letter m, as opposed to an en-dash, which was the same width as the letter n. Since most typewriters didn't have an em-dash key, most writers just hit the hyphen key twice to indicate an em-dash. MS Word is kind enough to switch a -- to a single long dash, but since AMCAS doesn't, I'm right back to the good old days of hitting the hyphen twice.
