I have had my flute for five years and recently since starting marching band It's tone and my ability to produce notes on the flue had gone down. To actually produce a sound I have to tighten my embochure a lot and blow a considerbale amount of air and it's constantly flat.
Should I get a new flute?
What's wrong with my flute?

Marching band is particularly brutal on instruments. Your flute is probably leaking.

Take it to a good repair tech; they should be able to fix it for significantly less than the cost of a new instrument.

I agree with TBflute.

Flutes are complex mechanical objects & require periodic attention to keep them working at their best. They are like cars - you usually don't realise how much the performance has deteriorated until after you've had them serviced. A competent repairer can pick up mis-adjustments before the player notices any problem.

It is a good idea to get them checked over - not necessarily a full service - every year or two.