I've been having elevated eye pressures for three years now. My vision has gotten worse. Pressures were 24/25 almost consistently. What could cause this? I was checked extensively for glaucoma and it came back negative.

Ocular hypertension. Ocular Hypertension doesn't always equate Glaucoma, although some eventually do develop glaucoma. Central vision worsening may not have to do with Ocular hypertension or glaucoma, and may in fact be refractive error. If extensive glaucoma evaluation involves visual field test and it was normal, just realize that almost 40% damage must occur before even showing up on a visual field.
Not always related. You need to see your eye doctor. Vision and eye pressure do not always correlate. Vision in glaucoma does not usually worsen until it becomes quite advanced. In some cases central vision can be affected early on, especially in younger patients.
2nd opinion. With elevated pressures and vision loss, glaucoma is a distinct possibility. You need a second opinion with an ophthalmologist with an M.D. Do this soon.