Avoid fatalistic view, you can do this!
Everyone has a bad day or weekend. Pick yourself up and keep going!
Losing weight is a mental challenge. No matter what plan you are on, the key is focus and control. You had a momentary slip that could get much worse if you do not maintain or return to self-control. You need to maintain control of your food intake and exercise on a regular basis. One moment of indulgence does not torpedo your plan to get more healthy.
You have plenty of opportunities to get back on track, and it is not too late to make today a success. Just remember, being defeated can be a temporary condition, but if you give up, it becomes a permanent condition.
The bottom line: Weight loss and maintenance is possible. The only thing standing between you and success is the bullshit story you keep telling yourself. Never treat today like the end, it is just another beginning. Consider each new start as a chance to take control of your diet and health.