Screwing up? Oh yeah. I switched programs 4 times...every single darn time I had about a year left. Took me 7 years to finish my BA degree. F's? You got it. W's? Of course! C's and D's? Yup. I think the thing that made the ultimate difference was that in my last two years, I had a combined GPA of 4.0, got three awards and really got as involved with as many extracurricular activities as possible while writing an undergrad thesis.
Trust me, I was a nervous wreck after I applied. I kept on thinking "they're going to take one look at my GPA and say, 'Pass! Next!'" There was so much self-doubt, especially because I rejected the offer of a lifetime from a university in the field I loved (and still do...long story as to why I dropped it in the end) and did so well in. I ended up getting into the program I wanted to get into for MLIS...and actually got accepted into all of the programs I applied for.
My advice:
-Work hard to raise your GPA since most programs require at least a 3.0 GPA.
-Be yourself and be true to yourself. If you've changed your bad habits, frame it in such a way that you do tell them you're a different person. Tell them that you know you need to be successful and state the qualities. Point out how much your GPA has improved. I know you can't read your reference letters since they're sealed, but if you trust your professors enough, you can tell them that you're sure that your professors would agree with how you've framed yourself.
-For the SOP, research is key. Make sure you know what is going on in the field.