This is actually a really important one. I have an aunt where eating properly made a big difference. As a start, I would suggest cutting out all pre-packaged food from your diet and cut down on (or even cut out) sugary/dairy/red meat products. Start making stuff yourself with fresh ingredients. It may sound like some new age thing, but it really worked for my aunt. She found that her moods were a lot more stable after she started eating properly.