You've got good, solid advise from these knowledgeable guys. I'm going to say the same thing, but just a little differently.

We have a paved 400' runway. Even with a trainer, I take 300-350' of it to take off.

I take off at only a 15 degree angle...very shallow. As others have said, you may be hammering the elevator too quickly and you're close to stall when you're airborne.

Full power and gently, slightly pull back on the elevator. Maybe only 1/4-1/2" from it's resting position. After it is 1/2 to 3/4 down the runway. Don't apply elevator too soon!

One of the above posts said that a properly-trimmed trainer will take off by itself. And it will! Wait until the last 50' and if it hasn't taken-off, apply a little elevator. A little!

Patience, gentle, gradual, smooth are the words you want to learn for take-offs.

They'll become your favorites. Promise!