The interventional studies which have clearly demonstrated an effect of diet on cardiovascular risk are the Lyons Heart Study (for prevention of a second event

The interventional studies which have clearly demonstrated an effect of diet on cardiovascular risk are the Lyons Heart Study (for prevention of a second event after a first) and the PREDIMED study (for primary prevention). The broad takeaways are that a Mediterranean diet may reduce risk, and more specifically, supplementing a diet with extra-virgin olive oil and some nuts seems to be beneficial. The magnitude of the effect of diet (for people without specific monogenic dyslipidemias) is not large, probably in part because most cholesterol in the body is produced by the body. In many circumstances – especially in the case of familial hypercholesterolemia – diet and exercise will not achieve an acceptable risk reduction. I would conclude by saying that your GP could have engaged with you more effectively to achieve a mutually agreeable outcome. Nevertheless, if you think "cholesterol == unhealthy", this is an acceptable gestalt understanding under the modern Western diet.