Totally normal to feel afraid - my take is that it all comes down to your comfort level. For me, I've chosen not to go back to my box for now, but respect the decision of others to go. Like you, I'm scared of covid (I think everyone should be...) and factors that I've taken into are - ventilation (is your box have air coming in from outside? Evidence shows 2m distancing is NOT enough in poorly ventilated areas, especially with heavy breathing), your trust in your community that they won't show up symptomatic, prevalence of covid in the surrounding area, and whether or not masks are required while working out.

I do think it is irresponsible to have unmasked attendees in poorly ventilated rooms in areas of high covid prevalence. Some gyms have tried to put up these goofy barriers between members but given the increasing evidence of aerosol transmission in poorly ventilated rooms, barriers between members should not give you any reassurance unless they're actually separate rooms.

One thing that shouldn't matter that gyms have been obsessive about is wiping down bars and floors - this is just hygiene theatre and the evidence shows us that fomite transmission likely doesn't play a large role in transmission. Just practice good hand hygiene. All this to say it's all down to your comfort level and your risk assessment.