My trumpet playing has gone down the drain lately. It's not just me having a bad couple of days either. My playing sounds horrible and my range is a lot lower than it was.

I'm guessing it's because I've been practicing my french horn more than trumpet lately. Does anybody have any suggestions as to how I might fix this problem? (and please don't say "ask your teacher," I don't have one, but as for telling me to go get a few lessons, I don't mind)

I know you're problem. I too had learned french horn while playing trumpet in high school and it killed my trumpet playing. Your having to make 2 different emburshers for the two different instruments, one smaller than the other. There are a few things you can do. 1) pick one and stick to it until you've come close to mastering it or learn more about alternative emburshers. 2) Keep playing both of them. Right after you finish practicing one instrument switch almost immediatley to the other. This will help your lips get used to playing the 2 different emburshers. You'll get better at both of them but a lot slower than chosing the first method.
The old fashioned way of learning one instrument at a time is the best in my opinion, unless you have just an excellent set of chops.

I never felt that, but probably because I started both 5 months within each other.

I don't play French Horn so I don't know what you mean. But my suggestion would be like jazzerjim says, just buzz your trumpet mouthpiece right after you play F Horn. Just buzz up and down a lot. Good luck with that!