Have more confidence in yourself. You can do it - and you will.
Dont think "I have to lose 5 stone" or whatever. That makes it seem like a mountain to climb and is dispiriting.
Rather, seek to change your lifestyle and then take it day by day.
Think of it like you are trying to change how you live (eating, exercise etc) rather than meet any particular goal. If you can do that, we find we achieve goals almost as a by product!
About 18 months ago, I was about 4 stone overweight. I am now a health BMI figure (albeit at the very top of the range).
I identified the "problems" with my diet - booze, snacks, portion control - and sought to fix them. We all have different issues in terms of what we do wrong diet wise.
Sure, sometimes I failed and drank too much or had snacks etc - but we all fail at some time, the main thing is that we are generally better. If we are, then the odd mishap is no big deal.
Dont try to live like a nun - all that happens is that you sicken yourself which leads to "giving up".
Allow yourself a treat once per week - chocolate, or wine, whatever it is - but ensure you are disciplined the rest of the time. (of course,a special occasion or holiday is different).
Try to meet the recommend amount of moving about per day. I had become very sedentary. Now I try to get out and about a few times a day, even if its wet or dark etc. Walking a mile is as good as running a mile, in terms of burning calories (1 mile = approx 100 cals).
What I found helped was visualising treats in terms of the amount of exercise needed to burn them off. A 45g bar of dairy milk has 240 cals. That means you need to walk or run nearly 2.5 miles just to get rid of 1 dairy milk. Is that really worth it, especially considering its not nutritious? (its OK as the odd treat, sure, but not every day).
Ditch large amounts of carbs (potatoes, chips etc) and instead fill up with generous amts of low calorie veg. You can really fill yourself up, with very little calorific penalty.
Be aware of your calorific intake vs the recommended amount and try to be calorie deficient each day to lose weight. If you eat healithy and exercise, you find you can make good "savings" which pays dividends.
Take it day by day, try to come in under the calorie amount and also exercising. You soon get into a routine. Your body will get used to smaller and/or healthier portions and you start to see results. When you do, you will be encouraged to try for more.
Its not that hard, we just convince ourselves it is. For example, if the daily calorie intake is 2000 calories:
breakfast: milks coffee, cereal and milk + piece of fruit: say 400 cals
lunch: tin of veg soup + 2 slices bread: say 400 cals
dinner: allow say 800 cals, including a good wedge of veggies.
Drinks lots of water too, rather than sugary juice etc - helps fill you up.
That is a healthy days eating (can still get 5 a day fruit and veg), but under the calorie limit, which some decent exercise on top will only improve on .
Have confidence. You can and will do it. You deserve it!