You dont say when you got the new cat and how soon you let them meet?
It can take months or even years before cats get on and they really need to be introduced properly over a long period and I would never leave them to get on with it.
Your two are quite different in age and could be the younger one is too younf for your resident cat.
You need to keep the resident cat allowed to raom the house and the new cat to stay in a room of its own and you need to start intros all over again.
After the new cat has been in the room and not allowed to leave for a few days, then you need to start swapping scents and move the new cat into a room that the older cat has been in and leave the door of the other room open so the older cat can go in there.
There should be no meeting of the two at all for at least a week and each day the young cat goes back into its room after its excursion in a different room.
Make sure you give the older cat lots of time and cuddles so she knows she is still loved but you obviously need to give the new cat attention and cuddles too.
If both cats are reacting Ok to each others scent during the exchange period, then is the time for first intros whichmust be supervised and you must be ready and protected to ensure that they dont fight and split them up if they do.
Dont give them much time together and leave them split up as before unless being supervised and its going Ok.
You keep on trying this each day and hopefully you will at least get a truce.
Do not ever leave them together until you are sure they can be trusted,
Loads of good luck and time means time, not a few days