Hi!
High weight and not using wash-out in the wing is the why your plane stalls!
I fixed similar conditions on several planes (planked wing) by taking a sharp knife and making 4-6 cuts (30-50cm long) in the balsa sheeting (on the bottom side) on the wing. Then twisting the wing ( the plastic covering removed of course) to get some wash-out , and then pouring some Ca glue into the crack.