Mixing tenses?

This is something I've always questioned about my writing, since its something I seem to do automatically and don't seem to put much actual thought into. But I tend to mix past and present tense in my writing. Here's an example from something I'm writing now:

"Yuuji had luckily been able to stock up pretty recently, so he probably had enough food for meals over the next couple of days. After that… he wasn’t so sure, he didn’t exactly fancy the idea of potentially starving himself, especially since Ji-chan was certain to notice if he was, and he really didn’t fancy pissing the old man off more than he usually does.

Yuuji sighs as he closes the door to the fridge, the stress of the day had seemingly piled higher than he’d originally realised, an odd pressure weighing down in his chest. He felt like he wanted to cry, to bury himself into a hole and hope the world forgot he existed. But, alas, he knew he couldn’t. it was childish to think that way—he needed to grow up, to bite back his feelings and be a man. Be strong."

The line "sighs as he closes the door to the fridge" is in present tense, but the paragraph before is almost completely in past tense. Is this a mistake I'm just constantly making? I didn't study English literature or any form of education in writing, and purely just use grammar that comes naturally to me so I have no idea if what I'm doing is actually deemed "correct".