undereating, really?

Tishrei

so I'm frustrated. I started here about 10 days ago, lost three pounds and have since gained it back.
some history....I'm 40 yo 5'5" and have weighed in the low 120's when not pregnant for the last 20-some years. then last year I gained 6-8 lb in 6 mo and spent 6 months watching what I was eating and walking daily to lose a total of 2 lb. gained it back a few weeks ago which is when I came here. I eat a plant based diet with the occasional piece of fish or chicken here or there and a spot of milk in my coffee. once in a while, sure I go nuts and have a carvel milkshake or a second helping of potato leek soup just because its good, but generally speaking I eat when I am hungry, I eat what I crave (I cook almost everything I eat, rarely eat out or eat processed foods) and stop when I'm done. I never really thought about it much.
for the last 10 days I've been measuring out my food, logging everything I eat and exercising more. I'm hungry and I don't much like that.
so now that I STILL can't close my largest skirt, I am frustrated. and then I read here that some people are saying that if you eat too little you wont lose and I'm thinking really? that just sounds too good to be true.
my diary is open so if anyone has the time and inclination, would you take a peek and tell me what you think? thanks, I appreciate it