My acne is really severe, and none of the OTC medications I've found work. Is there a prescription medication I can try?

Absolutely. The otc stuff doesn't touch severe acne. Nobody should have to have bad acne. I have had good success treating teens with topical clindamycin, step up to oral tetracycline if this does not clear it. For severe acne, a dermatologist can prescribe a medicine such as accutane, which has some risks but which is extremely effective.
Yes. Moderate to severe cases of acne should be evaluated by a doctor. A doctor can prescribe an antibiotic gel/cream, plus retinoids such as retin-a, tazorac, (tazarotene) or differin gel. Oral contraceptives (for women) and oral antibiotics are options also. Tetracyclines and Erythromycin are two oral antibiotics used in acne treatment. If acne is quite severe, a dermatologist might prescribe oral accutane.