I suck at this, too. I set timers or otherwise schedule meals. My husband is type-1 diabetic so he's grown up eating on a schedule and monitoring his carb intake his whole life. If you plan out your day's meals in the morning, with correct portions, and set timers to remind you to eat, then you will be OK. I use Chron-o-meter (Google it for the site) to plan my meals, but you could use other tools, like Weight Watchers' points system.
I basically eat the same thing every day now, with slight variations, and it generally works a lot better than when I was winging it. Having snacks near me when I'm focused on something makes it more likely I'll eat then, too. But I still usually miss my set lunch time if I don't set a timer -- and then wonder why I'm not focusing as well as I could. 
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Diagnosed with autism spectrum disorder 19 June 2015.