How do I stop worrying about losing muscle mass?

I think if I miss a meal or do cardio I start to lose muscle srs

By recognising that it's pure paranoia.

You will only start to lose muscle mass if you don't do any training whatsoever for around 2-3 weeks. And even then, a meaningful loss in strength will take much longer to occur.

cardio (although this isn't 100% true) and missing the occasional meal are more likely to slow muscle growth, rather than cause a loss of muscle mass.

If you're doing obsessive cardio and skipping meals regularly, then you will lose mass. Through simple reason of not enough calories going in vs going out.