I straighten my hairand i can never keep it sraight what should i use or do to help?

I straighten my hair everyday and it NEVER stays Straight. I have thickish kind of hair and my Straightener only goes up to 300 degrees. Ive used hairspray, heat protect spray, and a moose. I need to find a way that's affordable and that works (im only in 7th grade going in to 8th).If you know a way tell me i need to know. Thank you if you help ^.^