Sorry to hear that you have difficulties. For the sake of this game it likely wouldn't have hurt to also use the 'W' key for Jump since we weren't using it for anything else anyway and that would at least avoid forcing you to use your thumb.
For your personal play I'd recommend trying to use a controller if you haven't already. Having used both control setups for testing games I will say that controllers are usually a more ergonomic solution and easier on the hands.