The rule really is quite simple, you must have a valid visa to enter the Schengen area and your visa is only valid for the period mentioned on the visa sticker. On the other hand, if the visa is in fact valid on the 19th, entering a few days before what you originally planned is not a problem.
Entering before the start of your visa might seem less serious than overstaying after its end but legally it's the same thing and I don't see any room for interpretation here. If you try to enter Spain on the 19th, you should be denied entry and you probably won't even be allowed to board the plane to go there.
Unfortunately, you will therefore need to get another visa or postpone your trip. You should in any case try to contact the relevant Spanish consulate as soon as possible. They might be willing to alter the visa free of charge or they might demand a fresh application but they can in any case replace this visa with another one if they want to. The timeframe is quite short but even a completely new application should be doable in one week.