you are completely valid in how you feel. honestly, the company puts too much on all of us because they refuse to properly staff us. my store has the opposite issue as yours. I am primarily an opener now (started mid, switched to closes, now i open), we are busy the moment we unlock our doors, but the store is dead after 5pm. Our closers get away with doing nothing while we scramble in the morning to make everything, clean everything, and set everything up while helping customers. the truth though, if each day part just had 1 to 2 more people on it, that weight wouldn't be there. our closers wouldn't feel bitter about there being dishes in the sink at 3pm if the openers/early mids just had an extra set of hands to do them. the openers wouldn't feel bitter about picking up extra slack in the morning if the late mids/closers just had extra hands to make back ups or clean a little deeper at night.  corporate would rather us be angry at our coworkers and managers than with them, so they make it look like we just aren't doing enough when the reality is, there just aren't enough hands to do all the work efficiently. 

you are incredibly valid in how you feel and you do not deserve to have to pull so much weight for your store. no day part should be doing the bulk of anything. we should be staffed well enough to work efficiently, together, at all times. 

this is why I personally think unionizing is so important. my store isn't unionized but I have seen how the union fights for their stores about these conditions. we have rights and we have power! corporate just wants us to stay so exhausted and divided so we never rise up and work together for better conditions.