Thanks for writing to Healthcare magic,
I have studied your case,
There can be muscle spasm which leads to neck stiffness
and due to sports activity and horse riding it is getting aggravated.
you may need to do MRI spine and look for any nerve
compression occuring .
Take muscle relaxant
like USG and IFT will help you .
Continue good posture maintainance.
After you got relief you need to do particular neck exercises which will maintain painfree state.
Hope I have answered your query. Let me know if I can assist you further.