Heat red hot (the hotter the harder,) quickly remove from heat and dunk in water and leave until cool. This tempers it, it will be harder (and thus less flexible) but more easily breakable.

It depends on the material. Spring steel wire can be hardened to the point it snaps like glass.

Start out with stiffer wire. It's easier for me to bend stronger wire than to harden flexible wire.

You need to specify what wire you are using such as; materials including insulation, intended purpose, thickness etc Without these it is very difficult to help effectively.

Be aware that increased resistance to bending usually means decreased resistance to breaking.

What's the wire made of, how thick is it, and how do you define "very easily"? Some materials, and some sizes, just aren't going to put up much of a fight.