Welcome to the forum!. Are you using some sort of chip clearing? Dust collection? Air blast? My first thought is to feed faster. My second thought is what endmill specifically are you using? The broken one in the photo looks a lot like one of those ribbon single flutes that are meant for plastics. Those can work but are much easier to break. For the most reliable recipe, you will want to go with a more robust single flute with a shorter flute length. Carbide 3D sells the 274Z which is a good cutter for this. I have also used Amana and Datron end mills.
Take a look at the videos in this post of mine on another thread about cutting aluminum: