Wow, this is awesome for your first game. Congratulations! My first games were not even close to that lol
Although it is true the movement is too slow, specially when backtracking if you are a bit lost, the music helped there a lot. I love the main track (or the initial one, not sure if it is the main but my favourite by far), so it felt fine just going back and exploring slowly while listening the awesome track. Still an issue, but great job with the music. The SFX for the charge attack felt really clever from your side, but I understand that it is easy to miss. Art also goes super well with the theme and the music, congratulations on that.
The level design is rather clever to follow your vision of now showing any information to the player. However, a tiny bit more of information might still fit into your vision and make everyhting easier for players. Maybe show a little diamond when you get the upgrades, health bar for the bosses or better feedback that you are actually progressing through the fight and not doing something wrong. Reading the previous comments, I realised that I actually got a bug and the objects where not turning red if they are destroyable when the player charges the attack; that would have actually helped a lot and being a nice surprise. Sadly I missed it, but figured out it anyhow.
I got stump a couple of times, for example outside the bird boss, but overall really good entry :) Congratulations again!