I'm with Zoomie (up to a point! lol) on this one.
Tell her how important this is to YOU that you get your life together and going in a good direction. Tell her that because you're getting a little bit of a late start, you want to concentrate FULLY on your educational opportunity, without the distractions of having to work at any of the (easy, extremely lucrative and easy to Get) jobs you've previously held ~ you're telling her you want to stay away from shady characters, drugs ~ all of the things that you KNOW could contribute to you fucking up. This may or may not be exactly true, but you might be able to show her, in your grades, how things have been slipping, all 'cuz of your worry about finances.
It might impress her with your sincerity if, instead of a Gift, you asked for a Loan, and had some sort of repayment schedule in the back of your mind.
Also along the lines of Big Fat Plans: Show her how her money will help you to graduate as soon as possible, how you'll be able to take a night/weekend course or summer classes or those Crash Courses that you can do during the holidays.
Basically, just talk to her, and let her know that you've seen where you WERE heading, along with the fact that THAT life is something you now consider unacceptable, and that you are doing everything in your power to change that path.
p.s. BEST of luck to you with this, Sasha, because I believe that most of what I suggested up there is the absolute truth, and I sure enough am rooting for you!