How do I convince my mom to let me get a Cobalt Blue Tarantula?

Ok, I am 13, I own two tarantulas (G. Rosea and A.Avic). I take great care of them. I am planning on getting a new one so I can add to my collection (even though its not really a collection). I have chosen to get an H. Lividum (Cobalt Blue) sling, instead of an adult because those cost more. I have the money for one. Also, if you don't know what a sling is, its basically a "baby" tarantula.I have asked my mom MANY times, and she says i have enough. it kinda pisses me off. I worked hard for my A Avic and i will work hard for a Cobalt Blue. What can I do to convince her? She says I have to ******* GET RID OF ONE OF MY TARANTULAS TO GET A NEW ONE. ******* RETARDED BULLSHIT. Sorry, my keyboard has anger management.