Great job on making this game! It is inspired by Hollow Knight, if I can imagine? Maybe even path of pain? I do have to say that there is a number of things that could be done to make the gameplay feel smoother as a platformer.
As a game played with keyboard and mouse, it is awkward to hold down two movement keys at once--while also jumping, dashing, attacking, and swapping worlds. I would recommend taking the position of the mouse to determine the direction of an attack instead. I feel that there should also be indicators to hint to the player that they should swap worlds or show what path they are intended to take (this is why there are coins in Mario games).
I don't think that spikes should send you back to a checkpoint, instead knock you up because there would be a way to get out of the spikes (by attacking them). The acceleration on the player makes the controls feel a bit sluggish for a game that requires quite a bit of platforming skill. I would also add a double jump to make the game more forgiving, but that's more optional.
I think that there could have been more contrast between the two worlds, but I understand that is pretty restricted by the time. The forest could use a parallax effect too. Also, the lives on level 3 seem to be smaller than in the rest of the levels for some reason.
Overall, great for a first game, controls are a bit clunky but can be refined, art is great and the music is atmospheric!