Bit hard to say about that, as you're also using the mouse -- unless one of the mouse buttons is jump. The problem is that may hands are getting old, and doing something like moving right and then having to jump as well, is not only awkward but actually uncomfortable to do. It's a major issue for me in getting far into a game, as a certain point I will just give up. And I usually just play PC games with a keyboard and mouse, so if it doesn't work out then I will just move on -- especially for a game jam demo.
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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.