Change the way you think
I know that's easier said than done. Growing up for me was hard because I had an over-critical parent who used to tell me and my brothers we were useless, so I learned to think I was. It wasn't until I moved away from home that I was able to really start changing how I think by replacing those 'I'm useless, give up now' thoughts with more positive ones.
The thing is, if you believe
"You can't do this. It's impossible. You're going to be fat forever" then you are more likely to give up. I still have my moments where I think the same as you do and I want to give up (like recently when I was stuck in the 66s for a couple months), but I remember that losing weight takes time. Once the weight comes off, it will get easier to start thinking positive things.

You'll be fine. Just remember to stop yourself and change that negative thought to a positive one.
