Netball mums...lol  Ok start with the things YOU can control.  Is your mum going to change if you talk to her and explain the situation?  if yes try and just CALMLY explain what you just said.  If things don't change there then you are going to have to learn to just zone her out as it is not helping YOU.  Can you can change the fact that you don't have your normal coach coaching you?  No, so you are going to have to take what you can from this person as they are doing the best they can in a difficult situation.  Can you change the division you are playing in?  Seasons already started so I'm guessing no, so you are going to have to change your attitude about it.  Go in with the attitude that what a great opportunity to push yourself harder than you have ever pushed before.  Doesn't mean that you will be competitive in this division, however I bet your skills will improve compared to staying in the comfort of your old division.  You being tired and unfit. Sounds like you didn't have to work too hard in your old division, and the level of fitness required to keep up with these girls is pushing you past your limit.  Is this something that you can change.  YES!.  you can start by going out for a jog before or after school, doing footwork drills at home and try by doing X number and keep pushing to increase that number.  Do short distance sprint work, suicides, fast feet.   But this is something that only you can control and do for yourself.  When on the court and you get tired, try find a reason to push yourself past this point.  Is there someone who if they were there watching you, you would not allow tiredness to affect your game?  if so imagine that they are there at the moment you want to slow down.  the mind is the one thing that controls your body.  for some that means that they push their bodies past exhaustion and almost break down, for others it allows them to quit when the body has so much more to give.  which one do you fall into.  If its the latter then time to change the way you are thinking.  if its the first then time to listen to your body and get fit.

Change only what you can control, and accept the things you can't.

Hope that helps.  Good luck.