First of all, I love the sprites, music and sound. That's some good use of Gdevelop!
Longer-term, what are your plans for the game? If you're trying to sell it some day, what platforms would it run on? 🤔 I ask this because the extremely tall and narrow screen shape you have right now doesn't seem like it would fit on anything but a web browser.
I agree that it feels too difficult, especially for the start of a longer game. The default enemy appears to be armored whenever it's not shooting, requires all the player's actions to engage with (jet, move, and shoot simultaneously,) fires fast enough (in both directions!) to cover the entire platform in less 0.2 seconds, which is about the average human reaction time.
I suspect what happened here is you played the game hundreds of times while developing it, got really skilled with your own mechanics, and set the difficulty on what you personally considered hard, but fair.
This is a classic rookie mistake. (Take it from me, I usually make the opposite mistake and make my difficulty curve too flat!) But you usually want your game to be easy at the start, so easy it's almost impossible to mess up, so that new players who don't understand the controls can still shoot the first enemy. (That's how they learn to shoot other enemies!)
I recommend first enemy, just walks back and forth, doesn't shoot.
After about 5 platforms of different types with these walkers patrolling in different ways, introduce the player to an enemy that doesn't move, just shoots in a way you can avoid by jetpacking. Let the player stand on the ground for at least 2 seconds between shots. this gives the new player plenty of time to think about shooting back, jumping over shots, or just jetpacking past it. (But include a little zigzag armored ceiling so they HAVE to jetpack over the enemy, they can't just shoot straight up past it without noticing the shots.)
Then do a little obstacle course that mixes patrollers and shooters.
Finally, add an enemy like the Mets in mega man. (Those little yellow hard hat guys.) It's armored and doesn't move, by default. It shoots one bullet towards you when you get on the same level as it. And only after enough time for the player to react to the bullet, does it start moving horizontally.
Oh, and when the enemy is in its armored state, make sure there's some very visible juice when the armor deflects the shots, that's very different from the juice when they take damage (again, like the Metools in mega man, where the shots would make a ping noise and bounce off diagonally.)
An enemy like this that can do everything at once, and moves so fast you need to dodge, should be deep into the game, after the player has mastered moving, shooting, and reacting to bullets, separately and in combination with each other.
It's not a boss, though. A boss should move slowly, have lots of health, and switch up its attack patterns (but also telegraph its attacks so they're easy to dodge.) At least until the later levels.
Hope this helps! You've got some fantastic aesthetics going on, here.