Ok, everybody else ignored this one so I'll have a crack at it. (you drew the short straw and got me, sorry!)
First obvious blunder: you're calling the Input class in
_process and not
_physics_process - big red flag there. The former is just every frame, the latter is linked to the physics engine (with a set max framerate) which is obviously what you want given that you're working with the engine.
Don't call the physics engine twice in one frame. It's like a new girlfriend in that regard, it doesn't go down well to text twice before she's repled. Like this:
var power = 1000.0
var force = Vector2.ZERO
force.x += Input.get_action_strength("right")\
-Input.get_action_strength("left") * power * delta
force.y = if Input.is_action_pressed("thrust"):
force.y += power * delta
Obviously, I've tested nothing aside from this fine glass of red.
ps. you probably want an
is_on_floor for jumping but I'll leave that to you.
pps. don't you want gravity?