The "negative voice" thing is hard, and some days I think it's easier to reason with it (that is, make it shut up) then others. In terms of working out, I also struggled with the negative voice. This is what I did: when that voice would start in ("look at how big you got, this is hopeless, why are you even bothering," etc.), I would acknowledge it, and then "respond" to it by saying (in my mind) a variety of things, usually along the lines of: "Yup, I'm fat, and that's why I'm doing this," "It's only hopeless if I quit," "I'm fat, and I'm worth the effort not to be fat anymore." Here's the thing, and I know it's hard to believe now, but the truth is that the more you exercise/eat right consistently, the softer and softer and more infrequent that voice becomes. By accepting responsibility and taking action, you're no longer giving that voice much of a leg to stand on. It's a matter of taking that negative energy and focusing it to positive actions/behaviors. Not an easy feat for any of us. But possible? You better believe it.
I also agree with Vito; if the gym mirrors are getting you down - find an activity you like doing that's outside of the gym. I don't like the gym, so I have been consistently working out primarily at home for almost 2 years now. It works for me.
As a parting note, I was actually talking about the "negative voice" thing with my mom just last night. And the realization we came to is that, shockingly, things just don't get accomplished when you treat yourself like crap. If you constantly berate and belittle someone, chances are s/he won't make positive changes or even feel like s/he can change. Bottom line? Accept responsibility for the state you're in (whatever that is), accept that it is in your power to change it, and do it!
Start Date & Weight: 5/12/10, 250 (or thereabouts)
Current weight: 154.2
Pounds lost: 95.8
Goal Weight: 140