How do I avoid spending a lot of money on mobile games with microtransactions?

The most reliable way is just not to install those games in the first place, but if you're already playing, then delete your payment methods from your account right away or turn off payments in your phone settings. Psychologically it helps a lot: when there's an extra step between you and a purchase (entering a password, linking a card), the urge to spend usually goes away. Even experienced gamers recommend doing exactly this.

Second thing - set yourself a limit ahead of time and stick to it. If you decide to buy something occasionally, then "max this much per month" and that's it. Don't nickel-and-dime yourself with micro purchases, because those small amounts add up without you noticing. And remember: nothing in the game is worth going into debt over or regretting later. Very often these games deliberately create fake urgency - like, this offer is only today - but that's just manipulation.

And most important: if the game keeps pushing and making you feel bad without paying, if you can't actually play without spending money - just delete it. There are so many good games that don't require any spending. Your mental health is worth more.

Don't rely on willpower during a gaming session - when you need just one more thing to win, rationality goes out the window. It's better to set a spending limit through parental controls or ask someone close to you to put a password on purchases - that way you won't be able to make a spontaneous buy even if you really want to. By the time you're entering the password and explaining why you need it, the impulse usually passes.