I think it would be a good idea to listen to your mum and not cut your hair. You also need to really think about it for some time to see if it would be a good idea or not. You may cut it off without thinking the idea over and end up regretting it.
the real question is HOW long is it. If it's down to your knees you should try to convince her but it it's shorter than that it's best not to. If it is really long you could say stuff like it's getting in your way and all the cool kids have short hair and that short hair is easier to take care of
if it reaches your belly button then you need to cut it but if it doesnt then you should try dying it before cutting it before you can always dye it back but it will take a long time to grow back and you might regret it later.
Omg I love loonng hair, trust me, I had gorgeous, beautiful long hair, and I cut it like in December, to a little bit above my shoulders, I was crying, because I looked really weird and didn't really like it, trust me, maybe just go for like a 1 inch trim, other than that, u should keep your long hair! ;) and it's so hard to grow it back, mine still isn't growing much at all! :/ long hair is very pretty, especially curly or even straightened=)