Buy living life and experiencing it.
Let’s say you over-indulge in your spending. That will eventually result in a point-in-time when there is an expenditure that you’d want to take on, but you do not have the funds to do so. That is life telling you that you’ve over-indulged and that it’s time to scale back. This is akin to when you push yourself too hard in the gym, or diet too harshly, eventually something fights back. With training too hard, you’ll see lethargy, injuries, etcetera. With financials, you’ll end up owing someone money, or not being able to afford to do/purchase something (whether that is an experience, an item, or paying a bill).
Now, if you do not want to make these mistakes just to learn the lesson, that too is fine - but it isn’t necessary to err so much on the side of caution that you miss out on the richness that life has to offer. No-one can give you a general rule that will be guaranteed to work, hell, for all we know the next financial crisis might be around that corner and none of us will be having a jolly good time for years to come but for the sake of argument approach it the same way you would approach healthy nutrition meaning that rather than eating clean 90% of the time, spend wisely 90% of the time.
If a fancy coffee drink, such as a latte, is something that you’d enjoy then by all means you should treat yourself to one once in a while. You can set aside 10% of your money for these kinds of expenses every month, and whatever is left over at the end of the month you either let spill-over into the next month or you use that money to be generous (if that makes you feel good). Ideas: invite friends over for a home-cooked meal, invite your parents out to dinner, give to charity. Or, spend those remaining dimes and dollars on yourself. Maybe get some new chalk, it doesn’t matter. Use the money to enrich your life in a way that you see fit.
Whenever you spend money in an indulgent way, acknowledge it to yourself, by all means but do not let that stand in the way of your enjoyment of the thing. By doing this rarely, you won’t run the risk of habitualising it (exception: drugs, alchohol, …) and hopefully you might derive some extra enjoyment out of these years.
I’d never consider an expense in the grocery store to be unjustified if it’s put towards quality nutritional choices. Buy local produce, and locally sourced meats. Buy things that are in season. Buy things that taste good. Nutrition is what fuels you, and by extension, your success.
If you want any financial advice with regards to saving, I have but one: when your income increases - put the increase instantly towards your savings (stocks, etc.). Once you go “up” in lifestyle it is very painful to go back. This presumes a certain level of contentness with your current quality of life, of course.
Lastly, it is very easy to spend a little money, rather than a lot of money, and get very near the thing that is so expensive while paying a fraction of the cost. I like tasty coffee drinks too, but by buying some cinnamon, cardemom, cocoa-powder, and ground-up ginger I can get very near the experience of buying a campus luxury-drink and have an entire thermos of it.