I was voluntarily a vegan in my early 20s from 2001 to 2005. I too felt far better when I first transitioned to veganism, for several months. Over time, I gradually had a hard time keeping my weight at a normal level, and noticed that I healed slower from physical activity, but supplementation helped with that. The thing that ended up doing me in was an experiment to try to gain muscle mass (I had lost it while being vegan and working construction) where I started eating fish and eggs ended up having the unforeseen side-effect of curing my depression.

I'm not prescribing any diet to anybody, but it wasn't a great fit for me.

My father's development of heart disease was no mystery. He's a dumb redneck who went vegan because of a woman and the ensuing social circle that gave him what he always wanted, a sense of community. He's a shitty cook, except for meat, and so he ended up replacing meat with carbs and vegetable oils. Refined carbs trigger the liver to crank out triglycerides into the blood stream, who consequentially damage arterial linings and cause inflammation. His diet went from being a relatively healthy one that included animal products, but was a lot of game meat and fresh caught fish, to a diet that happened to be vegan but was shitty.

The religious zealots he was around told him that animal products were THE cause of heart disease and cancer and everything else. Total BS of course.