Why is the heart adversely impacted by chronic psychological stress, yet it benefits from routine physical exercise?
From the perspective of sympathetic nervous system

Exercise induced adaptations of heart

A common phenomenon in endurance athletes is the athlete's heart /athletic heart syndrome

Once athletes stop training, the heart returns to its normal size.

You must understand why this type of adaptation is happening in them:
Firstly the athletes train to produce work output at near maximal levels/ sub maximal levels of work done which cannot be physiologically achieved in a day.So the increased heart rate generated by the sympathetic stimulation is sufficient to fulfill oxygen demand of the body only at certain levels of work done by raising the heart rate
As the grade of work done nears submaximal level the oxygen demand increases in the body and the increased sympathetic stimulation (which also restricts the cardiac muscle efficiency by decreasing diastolic time,coronary blood flow) is no longer able to suffice it.Such high oxygen demand is responsible for the useful adaptations of the heart in the form of athlete's heart.Athlete's heart reduces the need for sympathetic stimulation to increase cardiac output .

During intensive prolonged endurance or strength training, the body signals the heart to pump more blood through the body to counteract the oxygen deficit building in the skeletal muscles. Enlargement of the heart is a natural physical adaptation of the body to deal with the high pressures and large amounts of blood that can affect the heart during these periods of time.


Comparison with chronic psychological stress

During stress the body may or may not have high oxygen demand (the principal stimulus for adaptation) because the sympathetic stimulation is psychological and not in response to physiological work.So the heart instead of adaptating gets afflicted with detrimental pathophysiological effects of prolonged sympathetic stimulation.

stressors’ contributions to diverse pathophysiological changes including sudden death, myocardial infarction, myocardial ischemia, and wall motion abnormalities, as well as to alterations in cardiac regulation as indexed by changes in sympathetic nervous system activity and hemostasis.

From the perspective of cortisol (stress hormone)

Effect of psychological stress on athletes

When we are talking about the detrimental effects of psychic stress it is important to note that it may override the cardiac adaptations and severely affect performance like,for example high cortisol levels in blood released in response to stress (physical/ psychological).Although cortisol levels in the blood observe a circadian rhythm in normal healthy indivituals, some reports claim that exercise reduces detrimental effects of high cortisol levels on the heart.However,stress may override exercise induced effects in athletes but this is a prospective study, subject to greater research

A prospective study found a clinically significant increase in overnight urinary cortisol:cortisone ratio during a high training load period in triathletes, who subsequently underperformed and reported fatigue

REFERENCE TEXTBOOKS:

Exercise Physiology And Ergonomics (An Introduction) Asis Goswami Academic Publishers.Pg-39 to 57

Essentials Of Exercise Physiology (4th Ed.) Victor L.Katch,William D.McArdle,Frank I.Katch pg-302-333