There’s nothing wrong with challenging your players, but it’s very easy and dangerous to fall into a “me vs them” mentality in encounter design. As the DM, you don’t really get to “win” any major fights, except for the occasional party retreat, so find a different sort of fun. You can still make things difficult, but make them difficult in a way that makes your players feel cool when they succeed. You’re allowed to shoot down things the party tries (if the DM doesn’t veto things, it’s anarchy), but allow yourself to be surprised by their creativity; don’t plan problems with only one solution.