Not sure about most of the meds bu my understanding is that the lamotrigine has some sedating side effects. So, perhaps then it isn't the medication.

How to treat the Tachycardia? I think treating it as Anxiety is the route to take. YOu need to develope coping mechanisms and strategies to keep in a 'tool box'. The 'Five Senses' approach and decatasrophizing aare great tools.

FIVE SENSES
when you are in a bad moment, imagine and visualize:
something to see - like a favourite phot
something to hear - like the sound of pasta shaken in a box
something to smell - like the smell of coffee
something to taste - like the flavour of your favourite gum
something to feel - like the touch of a towel

DE-CATASROPHISING
Make three lists:
1. worst case scenario
2. best case
3. likely
now consider the worst case and make a plan for if it happens