Brain chemistry can be a bitch to get lined out. Especially when parts of your brain aren't doing their job. I'm almost a decade post diagnosis and my quality of life has improved dramatically. It's not perfect and I still have to tweak things occasionally, but it's much better.

Everyone reacts to stimulants differently but the effects are usually noticeable pretty quickly. Keep taking your prescribed doseage and talk to your doc about it. I've never taken methylfenidate, but I've been on Vyvanse for years. Adderall was too rough of a ride for me. It took probably 6 months to get my dosage up to the right level. The main problem I'm having today is that it exacerbates my motor tics. Still working through that one.

Hang in there. If this stimulant doesn't work well for you, there are others you can try.