GW is based on the strength of your roster. As you get higher and higher level, it becomes harder to keep as many toons at or near max level. This means that everyone faces a relative peak in GW difficulty when they hit 85. As they progress more and level up more and more characters, GW becomes easier again.
But you have to plan accordingly and make sure that you also level/strengthen characters that will help you in GW and not just indiscriminately. If you do so intelligently, you can quickly get back on track with GW; if you do so poorly, then you'll continue to struggle with GW.
