well i think you have to make the choice to be more selfish!! that's step number 1 and not beat yourself up about it and feeling guilty.

you have to be de-stressed and looked after so you can be the best possible mum and wife,its going to effect everyone else in the family. so you need to be ok so you can look after everyone else!

i actually bought a yoga DVD online and it was like $9 and it only goes for 30mins and on a sat morning i get up and do it, even if the kids are eating there breakfast or having a nap.. its no stress but it you feel so much better and the one i have it has relaxation and breathing techniques that i use when im at the end of my rope!

also i find having a shower at night time with the lights off and just have a candle is really simple and give you time to unwind before you go to bed, its alot quicker then filling up a bath but make sure the DH has put the kids to bed!

with the food aspect i find i help out everyone and im left with no food or nothing organised for me.. so i have a draw at work with just nuts, musli bars and things like that to help me through the day and at least you feel you have something nice to eat! even if you have a fridge at work i buy some wholemeal pita bread and nice dips and i feel special!

hope some of these things help! even if im hanging out the washing and putting on some good music just trying to make it more fun and have a dance and get the stress out!

Look at taking care of yourself as a treat, not a chore and think of taking care of yourself as pampering! There is an excellent book that basically teaches you to take care of yourself, from all angles, it's called The Women's Comfort Book by Jennifer Louden. I love it and have gotten lots of great ideas for working taking care of myself into my daily routine. I love the above suggestions and agree that finding something to do that's fun will help you get motivated. Exercise is a big problem for a lot of people and I think it's because we grow up in environments where exercise isn't built into our routines. As kids we have PE, but that's when we're already full of energy and zipping around like crazy. Unless it's part of your job it's unlikely to get much exercise at work and it's usually the last thing people want to do when they get home. I don't know if it's affordable or if you have a Wii, but I love my WiiFit board. It has a training program that you can personalize and it keeps track of everything for you and has a virtual trainer to go through all the stuff with you. They have sports games, yoga, aerobics, balance games etc...and you can just do it in your living room and for as long or short as you like. I love the suggestion of a workout dvd too, there are a million good ones out there and you can do them at home, alone or with friends. I also like to put on cheesy dance music and dance around my living room (if no one else is home) or my bathroom (my hairbrush has probably been used as a microphone way more than it has as a hairbrush). There are lots of ways to get exercise into your daily routine that don't have anything to do with going to a gym. Get in the habit of taking a nice, peaceful walk around your neighborhood at night before bed (assuming you don't live in a dangerous neighborhood, of course). If you have a dog, it's a nice, safer way to get out and walk and your dog will love it too! I love taking walks before bed with the cool night air, it definitely makes me sleep better and I take the time to just wind down from my day and get some stress out. Gardening is also a form of exercise and you can do something like that if you have the time and space and want to do something productive while you are active.
It's hard to make dietary changes too and I would recommend adding healthier options to your diet rather than taking things away. That way you won't be depriving yourself, but you will be eating more healthily. I absolutely love Bumble Bars (available at any co-op or health food store) and I buy them in bulk, special ordered, so that I am stocked up. I take one with me anytime I go to work or someplace where I might be tempted to buy a snickers. It's way healthier that way. There are tons and tons of healthy foods you can work into your diet and the healthier you eat, the better you will feel and then you will only want to healthy foods and the junk will taste bad to you.
Most importantly, it sounds like you beat yourself up and are way too hard on yourself. Just take it a day at a time and one little improvement at a time, you don't have to do it all in one day! Good luck!

"If you want to tell people the truth, make them laugh, otherwise they will kill you." ~Oscar Wilde

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