What Dave said. NOW!

STOP ADDING ACID!

If you don't have immediate access to a acid demand kit, go to Walmart and buy out their borax (up to 20 boxes). Turn your pump on high speed, and leave it on. Add 2 boxes of borax every hour -- slowly, to the skimmer -- till you have a pH ABOVE 6.8. It doesn't have to be much above, but enough so you can tell a difference.

THIS IS URGENT! YOU MAY BE DESTROYING YOUR LINER AS I TYPE!

Given the way you are dosing, and testing, your actual pH may be 5.0 or even 3.0. If it is, it won't take long to do PERMANENT damage to your liner. And unlike liners bleached by high chlorine, the damage done by low pH is MORE than just cosmetic.

HURRY!