When I must eat at a restaurant, I have 2 secrets that help me avoid over-doing it. Here is #2:
2 - When placing your order, ask the server to bring a to-go box WITH your meal. If they forget, ask again when the food comes. Don't start eating until you've packaged HALF of your meal to take home!
Most places serve portions that are WAY too big, so this provides instant portion control. Plus, if you were truly still hungry, you could open the box to have more. But will you? No, I never have. And now the food's out-of-sight, so you don't sit there & keep picking at it!