Set a budget
Firstly if you are struggling financially then set a budget. Find out how much money you have coming in each month and how much you spend on essentials. Then you will know how much is left. You may wish to save some or use some to pay off debt but you can use the rest to treat yourself. Set out a budget as to how much you can afford to spend per month, week or day and stick to it. You will find that this way you should never spend more than you can afford.
You may find that every time you go out shopping you seem to come back with something. One solution to prevent you from doing this is to shop less often. You could perhaps just go out to the shops less or avoid going in those shops where you buy things that you do not really need. It can be a good idea to also avoid the departments in the shops which you do not need to buy things form so that you are not tempted to pick up things that you cannot afford.
Do not feel you have to buy
It can sometimes feel disappointing if you go out to buy something and come back with nothing. Try not to feel like that. If you feel like this you can end up buying something else to make up for the lack of spending and this can mean that you get something you do not need. Next time you shop you look for that thing you did not buy last time and you end up buying two things, when you only really needed one.
If you like having new things, then it can be worth repurposing old ones. Have a sort through what you have and see whether you might want to get some things out of the back of the cupboard to use again. You may even find thigs there that you had forgotten that you had or that are exactly what you need or want. If you find things you no longer like, then you could consider selling them to make some money to help afford new things or just donating them to a charity shop to make some space for new things. It is not always easy to get rid of things, but doing so can make you feel better and may even reduce your want to buy new things as you can see how much you have spent on other things that you have and do not use.