So what you ate ice cream? Guess what - I eat ice cream too! I had some the night before last. Don't throw away a "plan" just because you ate ice cream. You eat the ice cream, then you continue with the plan. No-one sticks to a plan 100% 100% of the time. That's real life
A bit of ice cream now and again won't harm your weight loss efforts anyway, you'll end up feeling deprived and sucky and you'll throw the towel in again and again. just make the mistakes and move on... It's what successful "dieters" do.