Do not worry if this right here happens to you because it is sure to happen to everyone eventually. But, it is also an easy problem to avoid so that is nice. Typically, what happens when you the plunger is hard to press down is the grind is too fine. When pushing the plunger down, in the first place, it should not be difficult to do so. If you need to exert a lot of force, something is wrong and if you force it you may end up breaking the seal. Go ahead and patiently plunge this round and the next time you go to brew more, try to use a coarser grind to see if that makes the process easier.