Need work advice

Started my first job ever and it's in retailing I have had about 1 1/2 shifts of training I'm still learning an still have much to learn but I only have 3 more shifts to learn everything before I have the entire place to myself an I'm alone on register for 8 hours..so far everyone has been very helpful an nice except one assistant manager who thinks she knows everything an don't seem to care to help when I need it....I'm not really liking it an I don't think I will be ready when the times comes to be by myself I really wanted to quit after my last shift bcuz I had a few angry costumers an I had lots of problems where I didn't know how to bring something up on the register on top of the register over heating and messing up..I think as my first job it may be a little too much pressure and I'm not use to working 32 hrs a week or standing on my feet for 8 hours..I dread every hour leading up until I have to go back an I want so badly to quit but it's only my 4th day so I don't know what to do