I need a budget, right?
Yes — but your budget doesn't need to be particularly accurate at first.
Budgets are important because they prompt us to make decisions in advance (when it's easier to make the right choice), and to see if we're succeeding or falling behind in our goals. (They also make us less anxious.)
The best budgets are perpetual works-in-progress: One study of 350 million transactions found that people who set unrealistic budgets still spent 22% less than people with no budget at all.
There's lots of easy systems for setting a budget: The 50/30/20 Rule devotes roughly half your monthly income toward essentials like rent and food, 30% for fun, and 20% toward savings. The Envelope System consists of putting some cash in multiple envelopes on the first of the month (rent, food, drinks with friends) — and when the envelope is empty, you know you need to stop. (You can also do this with multiple bank accounts.)
Or, there are online resources. Google Sheets has a monthly budget template. There's a bunch of other useful templates here. All those apps I mentioned can create and track budgets. And if you are looking for advice on how to start, I recommend spending some time on Reddit. (The first comment is particularly helpful.)
But what's most important isn't creating the perfect budget. Rather, you should be interacting with your imperfect, work-in-progress budget, and your behavior will start to change.