I have been adding a box of instant pudding to my cake mixes - I think it makes them more moist and flavorful but am wondering - does it soften the structure so much that it makes them harder to tort or stand up?

I'm asking because I am doing my first half sheet cakes for my sister's anniversary - I used a 12x18 pan 2" deep and used 3 mixes, 1 recipe of cake extender and 2 boxes of instant pudding. Cake looks fine, baked great, came out of pan fine, but is sort of breaking and collapsing on one end. It's wrapped in saran now and in the freezer and I think it will be fine, but I just wondered if cake is structurally firmer without the instant pudding in it - or is it just my inexperience with this larger pan?