There are several issues here.
First, what you claim on the W-4 does not have to match what you eventually claim on your actual tax return. If you want the maximum take home pay claim yourself on the W-4. If you want a refund at tax time, claim 0 exemptions. You mother will have no access to your refund. You will get it all. You file a separate tax return from her. It is not combined with her return.
At actual tax filing time, it is not optional whether you claim yourself. If you qualify as you mother's dependent, you cannot claim yourself. Since you are under age 19, you almost certainly qualify as her dependent.
