It's always a tragedy to see games like this that are so close to being great but with a few major issues holding them back.
The game looks great, I have no issues with the design, menu setup looks good, and the gameplay concept is good - but for me the game's engine just doesn't really hold up to it. It feels too choppy and more of a task than fun to get anywhere. There's a balancing act with stuff like this you have to take into account. If the aiming is shit and you know it, then don't make the game hard. Give the enemies less health so it's easier to get somewhere.
I had the same issues as most others with the cursor going off screen repeatedly, which was mainly due to the fact that it seems the cursor is above the crosshairs. Why not make them the same?
The firing seemed a bit odd as well...there were like two crosshairs and it was difficult to see where I was shooting exactly, but for the most part it was manageable.
For the most part it just seemed unmanageable. The aiming was odd and the gun I was given felt underpowered against the army of asteroids and ships that were attacking. I see where you were going with the gun overheating, but like I said before it's a balancing act between the game mechanics and the difficulty. If there's any issue with aiming or shooting, take the difficulty down a notch at the very least so the game feels manageable. I think if the enemy ships didn't have such a ridiculous amount of health that this would have been far more enjoyable for me.
My totally honest advice would be to beef up the gun a bit or reduce enemy health, and maybe take a look at the crosshair/aiming issue. I think with those two things tweaked, this game would be a real winner. Overall I just had an 'uncontrolled' feeling of the ship and that's why I didn't enjoy it as much. Hope this helps.