I feel for you, having your first grow go apeshit is pretty devastating.
The PH down u bought is the opposite of what u need, you are strugling with a low PH soil, and PH down is used to lower PH.
Honestly i would make list of mistakes/lessons learned, post it up and have the forum help you rectify for your next grow. Start a diary a few days before you germ, and give all the info u can in regards to your setup (soil brand, nutes, etc.) type out how you have planned your grow (nute schedule, watering schedule). That way it will be easier to give advice and your chances of succes the second time around will sky rocket
Anyway, my two cents.
Anyone with more growing experience feel free to tell me im wrong, as I have not got alot of growing experience, only a solid few years of research and forum lurking knowledge

ATB whatever you decide mate
EDIT: wasnt there a picture of a PH down powder a few mins ago in this thread?