You are seeing why foldforming is generally done on 24 gauge/.5mm or
thinner metal, aren’t you?
When I need to fold form heavier gauges, there are two things I do
to make it easier to open:
1- Don’t confirm the edges all the way closed. Only the fold needs
to be well-confirmed (For those new to fold forming: confirming a
fold is like creasing a fold made in paper. We confirm by hammering
next to the fold, or running it through a rolling mill.)
2- Once you’ve got a little bit of an opening, put a wedge in a
vise, put the opening over it, and mallet the metal down onto the
wedge, to open the fold. Malleting this way give you more force and
leverage than pushing an opening tool into the opening.