Suppose you have a spaceship filled with negative mass fluid, making its total mass equal to 0. That means that it receives no gravity drag and can reach the speed of light. Problem is, the mass of the ship will not remain fixed. The more fuel is depleted, the more negative mass will outnumber positive mass, making the ship accelerate backwards.

Even if the ship doesn't use fuel and it uses a solar sails, how will positive and negative mass remain the same given that space dust particles will pierce and tear the sails?