How should i deal with takeing my adderall for ADD?

the doc prescribed 30 mg ir adderall for my add adhd .im also on xanax for anxiety..ok so the doc prescribed the 30 mg tab once a day;...but im a big guy i wiegh like 260..and the adderall doesnt last all day ..so ill be in the middle of something and come down from it and ill loose my focus and get down.so ill take another .and when that wears off i usualy take like a quarter of a 30mg adderall wish is like 11 and a half mg.and then thats it. that seems to work.it gets me all the way threw the day..but at the end of the day when it completly starts to fade i experience like micro mood swings..ill be fine then sad then fine then sad then ok then ok then sad then anxious then fine...so so ill take my xanax xr to helo with erritability and anxiety and slight depression..it works..but i dont kno if that right. n do i keep takeing it to get it worked up in my system..is that the problem .do i have to take it for a while to see if its gonna work for me..im not excessive when takeing them.i just take enough to get threw the day...wich from what ive read isnt much diff from alot of ppls doses..but so do i let it build up in my system..or should i not take it..cause we tryed anti depressants befor and they dont do anything .....so im just curious i wanna figure it out so i can find the perfect dose and not have to worrry about wether im gonna feel like crap in ten hours or if ill have moodswings..there not big moodswings.none else notices it .there just like random..and only when im on the adderall.im 21 by the way so i dont kno how to talk to my doc so he doesnt think im a pill seeker...cause im not...i just wanna get threw the day without being miserrable or lazy ....plz no stupid rude pointless comments. if its not answering the question dont answer.