I regained 20lbs back in March/April, by going way off plan. When I finally decided to weigh myself again, I thought "Wow this really sucks.".
Then I restocked my fridge and cupboards with diet friendly foods, starting walking again, and now I'm back to where I was, only stronger this time.
Trust me, it gets easier and easier to get back on that horse! It may seem like the end of the world right now, but it isn't. Overall, you've lost a lot, and if you were to chart your weight you'd see a downward trend.
But you have to keep going to see that trend!