Buggy
Here's a list of bugs to work on that I noticed (welcome to the life of a programmer :P. It gets easier eventually, though.)
1 - your guy starts below the level of the ground sometimes, so he can't move sideways until he jumps. He should start either at the level of the ground (if that can be done easily) or a little bit above the ground so he just falls to the ground at the start of the level (if he can't be made to start right at ground level easily)
2 - you can only jump while you're moving sideways, and can't jump if you're standing still
3 - when you jump high enough to hit the ceiling, you lose sideways momentum, which makes it tough or impossible to jump over stuff. Ideally, you just want him to lose upward momentum when he hits the ceiling, which would require your code to distinguish between ceiling collision and wall collision and handle them differently. Or if you're lazy, just don't let the guy jump high enough to touch the ceiling. Or if you're super lazy, say that the ceiling is covered with superglue and it's your job as a player to not touch it. But then you would still need to make jump height change depending on how long you hold down the jump button.
4 - just make the guy die if he goes outside the play area, or put a wall around the area, instead of saying in the comments to not go outside of it.
Once the mechanics are working nicely, focus on level design and better graphics and some sound if you feel like it