A really interesting game! Love the concept. However, it hinges too much on the hypnosis working by itself. The game state and mechanics aren't doing anything to get me into the right frame of mind, let alone trying to make me feel confused enough to mess up. In fact a player can get very far just randomly clicking as long as they leave the right hand card alone.
Suggestions:
1. Have a randomized upper limit. 25, 43, 36, etc. If the player must remain below a number, then they must think more carefully about which cards to play. If they go over, then they forfeit their score for the round. Mental taxation is desired!
2. Have an occasional smaller 'target' number that must be landed on directly before ascending past it. Same result if the player blows through it.
3. Simple debuffs that make the values of the cards different. Such as a minus 1 to all card values, or even and odd numbers are swapped, or just saying something like 5 is now 8. The original face would be unaltered, of course! It would be up to the player to remember how their cards have changed.
4. Secret traps! Very very occasionally a debuff effect is applied secretly to a card as it is played. To keep us guessing and make us doubt our calculations...
I could go on forever. I get a game design itch from games like these because they are so close to working. Getting the challenge level right is very important for games that are trying to induce feelings through its game mechanics, esp if the player is meant to feel overpowered, tricked, or helpless. I doubt I will personally ever be hypnotised by anything, I've been trying for far too long to believe otherwise. But I'm sure that by putting your best foot forward with your game's mechanics will give everyone else the best chance they could have.