How can I persuade my dad to let me spend more time with my mom?

I'm 15 now, and for a solid 13 years, my dad has had the majority of custody over me. According to the courts I'm allowed to see my mum every other weekend and Wednesday. All my life, practically, I've always wanted to be with her more. My mom and I have a very good relationship, and my dad doesn't seem to understand this. Some things happened when he married with my step monster that really set me off on this subject [he's divorced now] , but now it's just that I'm older, my mom now has a stable residence that's close by, I'm comfortable over there, etc. I don't want to have to go to the courts to change my custody; it would be difficult, and would probably turn my dad against me. I already know for sure the courts would side with me. I just want to be able to give him outlined reasons as to why there is absolutely no valid reasons for me not to able to spend a few more days over there. I feel like he doesn't understand me, and doesn't respect the fact that I have two households, families: practically two lives. I'm just asking for leniency. Advice?