The system I am designing is based on ten-sided dice pools. The success of a die roll is determined by a difficulty number which defaults to five. Rolling higher is a success, while rolling equal to or lower means failure. Since this is a dice pool system, the number of successes are variable.

My main difficulty is in describing bonuses and penalties to the difficulty number for the roll. It is easy to describe this for the dice pools themselves - a +X bonus to the dicepool means that many more dice are rolled, while a -X bonus means that many less dice are rolled. Right now, I am describing modifiers to the difficulty number as "X more difficult" and "X less difficult." X more difficult means the difficulty number is increased by X, while X less difficulty means the difficulty number is reduced by that much. This does not seem to be the optimal wording for me, although it is unambiguous.

1 and 10 have special results, so I cannot simply describe the modifier as "+X" or "-X" to the dice rolls. Even though it is possible to get into the wording of rolling a "natural" 1 or 10, I would prefer not to go there.

I am hesitant to simply denote this "+X difficulty" and "-X difficulty" because, for myself, the gut reaction to a plus sign is to think of it as being better for you, while my gut reaction to a minus sign is to think of it as worse. This simple point made it very difficult for me to understand AD&D 2e, even though it was not too different from D&D 3e. It sounds silly, but simple pluses and minuses really did make it hard for me.

I am also hesitant to simply reverse the definition of a success such that one desires to roll under the difficulty number for similar reasons to the above. In this case, "+X difficulty" would indeed be good for you, but the player will then have the gut reaction of thinking that "+X difficulty" means that the rolls are harder, when they in fact became easier.

I decided to try out wording the difficulty number modifiers as "+X difficulty" and "-X difficulty." I can't believe I missed something as simple as that.

Glad you managed to figure it out. Alternatively you could denote +/- die as +1d10 or such. You might want to in fact do that anyway, to avoid confusion with difficulty.