i don't know if i've posted this is the right section but ah well.
i'm quite shy & submissive..if someone says something to me that i don't particularly like, i'll take it in, not bother to fight back or stand my ground.
recently, the girls at my work have been making little comments (they are around 23/24ish, i'm 18). what they say isn't paricularly nasty, but i don't usually say anything back. i mean, i'll just agree or whatever. anyway, this weekend, i was at work & when i was helping some customers out, two of the girls started sniggering and laughing..i thought nothing of it but then they made comments like 'you sounded confused when you were with that customer, like you didn't know what you were doing..'. and then later, the two girls were talking about something & i asked them what they were talking about. they just replied with 'your too young to know!' i feel like they treat me like a child, and are very patronising when they talk to me.
as well as this, i feel that just because i am the youngest person working in the shop, the girls take me for granted. eg. if customers are waiting at the till, they'll be like 'get on the till!'
when comments like these are made, i just do as they say. but now, i feel like I DONT want to do what they tell me all the time, i DONT want to be taken for granted.

its just getting me down because i feel angry whenever i'm at work because of the way they treat me. i really want to stand my ground & as much as i force myself to, i never do in the end and just take whatever they say.
what can i do?