The IR is short-acting, only meant to last 3-4 hours. You may have more success with a long-acting formulation that you take only once.
Age and weight have no effect on the stimulant dosage an individual needs. That med and dosage depends on neurochemical needs, metabolism, and genetics.
It definitely sounds like you need to find a different clinician. You should feel heard and respected.
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism