Well what are you having for dinner (like what will be served?)
I know exactly what youre feeling, Ive gone through it MANY MANY times lol. It doesnt seem to get easier...I can tell you what I've done though, but its not really a good thing to do. What I did a lot was to eat very little (i ate SOMETHING, just not a lot..lunch and breakfast totalling 500ish Cals) and then ate without worry at dinner. This way, if you have a lil of everything (even if you dont and just eat what you like in normal portions) you definately dont have to worry about going "over".
Another thing you could do (which works really well for me) is to copy what someone else is eating. Take your mom, for example, and fill your plate like she fills hers- you can even take out some things that she has, making it even less then hers. You know that she can eat normally and she will not gain weight from this meal, so you wont either.
If you find out what you're having, you can plan what youre going to eat and make it reasonable. For example, if youre having steak and mashed potatoes and salad: Plan to have a palm sized portion of steak (easy to visually measure), a scoop of potatoes and lots of salad. The steak will be about 200 Cals (palm sized), a scoop of potatoes (lets say its a half-cup) will be 100 and the salad is probably 50 (up to 100 if you have a lot). You could have a drink but its up to you. You could always stick to water, a diet pop, or have a glass of juice/milk/can of pop (150 cals) Total dinner? 550, including the big salad portion and a can of pop. That's not THAT much!!
So you can have your dinner (pretty reasonable cal too..), appear to eat normally, and not get harassed at the same time. Hope this helps!!