I'm useless at keeping a diary about anything. I've also heard that posting regularily on a support forum also helps and there are some dedicated to weight loss - but I'm sure we're happy to help. Write yourself a goal, eg I want to fit into size ? and feel comfortable. And write why you need to meet the goal - I want to feel healthy and good about myself?
Avoid goals that specify a specific weight or target kilo loss - for some reason (natural weight fluctuations?) they don't help, they just make you sad when you get heavier and irrationally happy if you get lighter - but like I said before - no connection to your health or reality ie drink 500ml of water - gain 1/2 a kilo. Sit down for a dedicated session on the loo in the morning - lose it again. That time of the month - gain a bit. Hot day - lose a bit, nothing to do with your health and fitness - it's just natural.
Nobody is perfect, everyone is going to have their bad days and sometimes fall off the wagon. If I'm feeling like I need to cut back on food, I usually have one day a week that I give myself permission to eat what I want without feeling bad about it. If there is a day when I can expect to be eating a lot of food eg a night out at a good restaurant - I make it that day, otherwise for me it's Fridays.
You need to have other food at work that is ok for you to eat. Do they make the cakes and things at work? Could you organise to have something relatively healthy like date and walnut slice with wholemeal flour? Yummy and reasonably good for you even if it is a bit calorific - it's nowhere near the lemon curd tart calories.
Take carrot sticks, cheese, walnuts (not low cal but good for you). Eat a bit more protein - fills you up. Drink a 300ml glass of water and wait five minutes before caving in to temptation. Wait 20 minutes before deciding to have a second helping. Cut the naughty food up into little bits and have some every 10 or 20 minutes - try to drag the time out that it takes you to eat it.
And every time you fall down, just get up and start over. Don't use failure as an excuse to quit - it's a learning opportunity - what tempted you that time and how can you reduce the problem next time. Hang teatowels over your side of the display cabinets so you can't see the stuff?
Oh and find another 30 minutes every day to walk the dog. Park a little further away, get off the bus a stop further away...