Why does one text from them change my whole day?

When attention arrives irregularly, your brain starts treating it as a reward rather than information. Each message brings a rush of relief, and the gaps bring anxiety, so your mood ends up tied to their phone. That's why a single text can lift a whole day and silence can wreck one. The way out is to shrink the reward: fewer checks, more of your day that doesn't depend on them, and noticing the pattern each time it happens.