Nice job with the game, it was fun to play.
It was a little confusing at first. It took me a few deaths until I realized what I needed to do. You may want to put the player in a safe space starting out. Let them play around with the controls for a bit, then once they get high enough, start pushing the spikes up.
Having to choose the difficulty after each restart is a little off-putting. I would just have a restart button, then a smaller button under it to change difficulty. I personally wasn't, but sometimes players feel insulted if they're repeatedly asked to choose their difficulty.