I've found the following to be helpful in trying to ward off stress, mood changes, anxiety, and mild mania without medication. I have found no good wards against low moods or depression.
If you find that you are tense, or are being stressed out, or things are happening too quickly, or it is all too much, take a few deep breaths slowly in and slowly out and try to relax your muscles. It provides partial relief.
When things get overbearing, take 2 minutes out to decide what you are going to do for the next hour. There is nothing in the world that cannot wait for 2 minutes and the planning of the time relieves you of some stress. Write down what you intend to do if necessary.
Over the next month or so, pick one or two things you don't like and want to change. Then try working on it to prevent it from happening. The feeling of control makes you feel better. You can pick odd things. I've picked on driving less than 80kph (instead of at 120-140kph) as one of my goals. It makes me calmer overall to do so.
Do the following every day: Relax more. Stop feeling guilty for the things you did not achieve. Stop feeling anxious about the things that you are not achieving. If you have a partner, spend 10 minutes lying quietly relaxed or cuddling with him/her every day. (It's therapeutic for them too.)
If you can make the time, exercise. Whatever you like. Exercise makes you feel better. Don't forget to have fun doing it and don't feel guilty if you skip a day or ten. Just smile and start again. Don't let the gung-ho exercisers make you feel inadequate. I've found swimming, walking, or medium intensity aerobics to be fun, and stretching at the end of them to be very relaxing. With the aerobics I felt really stupid for the first 4-5 classes until I got the hang of it, but after that it was fine. Whatever you choose, make sure it is something that you can still achieve even on a bad day. With swimming, I set the minimum number of laps to an amount I can achieve even if I am feeling lousy. That way even on bad days I feel as if I have have accomplished my goal anyway.
Monitor the signals that warn you that you are becoming stressed or overreacting. Calm down using deep breathing or anything else that works.
Try to overcome the guilt / stress / embarrassment on the things that have not been done. Start over afresh. Get the people close to you to just lay off on your case and tell them it will take you a while before things are back to normal. Don't overwork trying to get things back to normal - things will happen when they happen.
Get your partner and those people close to you to tell you - often - that they love you. I'm not exactly sure why this works, but it makes me feel better.
Talk to other people. It is the only way to let them know how you feel. Find a few close people and tell them how you honestly feel. This is a therapeutic thing (but remember that relationships are on line here, so still be tactful in what you say).
I have no recommendation for diet. My experience is that diet follows mood. I would suggest that you try anything you feel as long as it is sensible.
A warning. If you really are bipolar, all of the above will be very useful, but may not be enough to prevent you from destabilising. Please hold that at the back of your mind if you stop taking your medications, and please be aware that you may have to go on drugs to stabilise.