Struggling with Lab Dynamics and Mental Health
Is this a normal part of graduate studies?

No.  Good research management would clearly delineate each person's responsibilities and assign appropriate credit.
However, nothing is normal this year.

Should I just let this all go and move on enough to get that diploma in hand?

The purpose of a masters degree is for you to learn.  Focus on that.  You are not in a competition with other students.

How can I deal with the mental load?

Consult a licensed mental health professional if you feel distressed.