I had a challenge with the texture of animal foods, and my former strategy was to cook the heck out of everything. I had to identify textures and develop chewing strategies in order to eat raw meats.
In my case, I found that I used "tough" to mean several different things: gristly, rubbery, fatty, chunky, etc. My reflex action was to want to spit out the food. Then, I started reasoning through this reflex, and I was successful in learning a new way to chew each texture.
To me, heart is rubbery but penetrable, a medium-to-easy job for the molars. I've found lamb meat to be a good training ground for chewing different textures. I find lamb heart in halal markets.
