Sounds as though they have you 'pegged' to be the shop junior. How do you think that you come across? - as positive and trying to prove yourself or, resentful and angry about being disregarded?
If you are positive and show creativity, enthusiasm and drive, you will be given a bit more responsibility within a month and it wil lgrow from there, so long as you do well and stay self-aware about how you come across to others - not fake, not impatient, just easy going and professional!
If you are withdrawn and resenrful, it will come across loud and clear and would not encourage any employer to give you more responsiblity.
I say all of this to be constructive and am not totally aware of your working envoronment. It sounds like a 'favouritism culture', where you may have to fight a bit more (constructively of course!) to get noticed.
You can rise above anger, prove yourself and see the benefits in time or, look for something else that will make you happier, if you feel that you can't / dont want to put on a bit of an 'act' for a while. If the job is right for you and you do go for it, the rewards will come, I assure you.
Either way, good luck to you. You sound an intelligent person who has ambition, so in the long term you will be just fine. Dont worry, It can take a while to settle into the right career 'grove'.
Be Happy, try hard.