I agree, don't look at the overall big picture. It's daunting and frankly scary. Not just the sheer physical extra 100 pounds and what its doing to your health, but who will we be if we're not the fat daughter, son, sister, mom etc. All of that can be scary. Focus on each 5, 10 pounds. In my head, I chant, regularly, "Losing 5 pounds doesn't change who I am to those who love me", now, losing 100 would be a wake up call, but 5 or so at a time? No biggie, just do it 20 times

Focus on each extra step you take during the day, each snack or meal where you choose smaller portions, more healthier choices. Small choices are things like, instead of parking in that spot 2 from the door when going shopping, park half way out in the lot, then the next week or so, park at the last spot in that row, etc. Choose a glass of water over a soda when you hit the vending machine, when you cook dinner, instead of adding butter into your veggies, add a bit of water and steam them. Those small choices all add up. I started at my highest weight of 269 and it's a bunch of small, barely noticable changes over a period of time, that has brought me to 220. You can do this! You just need to practice, and remember, slipping up, falling off the bandwagon etc, is NOT the end of your new way of eating and life, there's no sense in battering yourself, frankly, to many other people are willing to make comments on how fat lazy etc you are for you. Use it as a learning lesson and move on.
**Disclaimer. I know not all fat people are lazy etc, but that is how many people see us and we've almost all heard the comments***