Hi Jules,
I also had the opposite problem; the combination of chemo, surgery & radiotherapy dropped me down to a very unhealthy weight. However, I have noticed a couple things that hopefully may help.
One, eating patterns are often a reflection of internal states. So, for me, when I am filled with drive, passion for life and purpose then my diet seems reflect this inner energy and I put on weight (good for me).
That loss of motivation (called an over balance of Kapha in Ayurvedic medicine); not just in food, but often reflected in every thing we do, be it filling up our time with useless things or the pilling up of clutter in our room, is also connected to something deeper. However, if we can force ourselves to start to make changes on the outside they often reflect inside. So for me when I'm going through a lazy mood then making myself wake up early, do a yoga routine and being active through the day eventually counters the mood and after a couple days I'm back on track.
Also another thing I've seen can be really helpful is slowing down eating and increasing your awareness. So only eat sitting down and aim to chew EVERY mouthful 10-20 times; you'll be surprised how much less you end up eating :-)
Hope something here helps,
Love & light.
Astley