Your symptoms sound very much like nerve damage. I've never tried a 15Hz myself(4Hz) though I often think of the potential complications that may be caused with prolonged use.
For what it's worth, I'm sorry to hear about your experience. Though if it's any consolation, there's a likely chance that your symptoms will subside over time. And I say this based on the fact that I too suffered neurological damages due to Antibiotic use, which resulted in pain and difficulty walking for almost two years. Needless to say it was quite concerning that it took so long for things to improve, but they did. And so I'm thinking there's hope for you yet.