Accidently mixed instant and cooked pudding packages
update: It seems that the salts used in instant pudding are heat stable (one turns into the other one at 450°C), so it's possible that it might work by following the cooked directions ... but I'd advise a slurry as a backup.

You're going to have to add something to fix it.  Either more gelatin (to have it set cold), or starch to have it set when boiling.

I'd probably go with the hot method -- you'll bring it to a boil, which will screw up the setting properties of the gelatin.  I suspect that you'll want to let it boil for a minute or so, to make sure the gelatin isn't going to set at all (and possibly make it too thick, or interfere with the starch).

You didn't say which size of boxed mix you're using, but looking at a few recipes, it seems like you want 3 to 4 TB of cornstarch to set 2 cups of milk.  Use that as a basis to scale your recipe.

(updating instructions per DebbieM's comment)

So : 


Mix cornstarch (1.5 to 2 TB per cup of milk needed for the instant pudding) with some (less than 1/4 c.) of milk in a jar, seal and shake to make a slurry.
Stir in the cold milk (the sum of what's called for on both boxes).
Heat to a boil (see notes from cooked pudding package about stirring, etc).
Check to see how well the pudding coats the back of a spoon.  (wipe a finger through it to get an accurate estimate of how thick it is)
If not thick enough (note: it will thicken more when cooled), add some of the slurry, and let it boil for a minute or so (keep stirring or whisking during this).
Repeat 4 &amp; 5 'til you've achieved the thickness desired.
Follow the cooked pudding package instructions from there.


If you don't have cornstarch, I've seen some recipes that use flour (about 1/4c. per 2 cups of milk), but they're always chocolate ... I suspect it's to cover up the raw flour taste.

If you have unflavored gelatin, you could also bloom some in some cold milk, and then mix it in with the cooked pudding as it's cooling to try to achieve the desired thickness, but too much might end up setting like a blancmange.