Ferrules come in 2 sizes, meant for 4mm and 5mm outer diameter housing. Are you, by any chance pushing 4mm onto 5mm? Otherwise, it is common for thoughtful, gentle and delicate to beat brutal, no tools required
. Make sure there is no fraying to the outside in the plastic on the cable. If there is, cut it off. Gently slip on the ferrule, twisting around if necessary. Using the old housing, you should be fine too.