If you find it hard to stop eating once you've started, there is a trick you can try which makes it quite difficult to overeat. All you need to do is eat as close as possible to a time when you need to go out. For instance, if you have an appointment or when you need to collect children from school.
Decide how long you need to eat your lunch, say 20 minutes. Then allow however much time required for preparation and add the two times together. For example, supposing your lunch takes 20 minutes to prepare and 20 minutes to eat, start preparing 40 minutes before you need to leave home.
This way you can't continue eating if you have to leave. Just don't grab anything to take with you!! Hopefully, by the time you've returned home, temptation to overeat will have dwindled.