I have ectomorph body-type
The somatotypes is a very old theory, and one that has been under a lot of scrutiny in the actual scientific environments. It is deemed suggestive, but not conclusive.
But that said, I understand what you mean when you say you are ectomorph, so if nothing else, at least it serves as a starting point for this conversation.
which means I have high metabolism
That's the wrong interpretation. The reason why "ectomorphic" people have a hard time gaining weight, is not because they have a high metabolism, it's because they have a low appetite.
You're simply not eating enough.
And I know what you're going to say.
"Dude, when we have tacos I eat like 6 huge shells and chug a litre of coke. My appetite is huge!"
But this simply boils down to the fact that your appetite isn't measured by your largest meal of the week. It is measured by how much you eat overall during the course of a week. Hell, even that is too short.
Instead of looking at how much you eat in a day (which can vary by large percentages), you should be looking at how much you eat in a month. Because when you eat super-huge meals one day, you'll just be eating less the next couple days, and you're back where you started.
At the end of the day, if you want to gain weight, you have to eat more calories than you spend over any given period of time, preferably measured over a week or longer.
On the same coin, if you want to keep your current weight, you need to consume and spend equal amounts of calories.
and lose weight very easily
Yeah, that's fair enough. But don't blame your metabolism. Blame your appetite.
Remember, it's not supposed to be easy to gain/lose weight, when your body type dictates something else.
You know that fat people have a hard time losing weight because their appetite is big. Skinny people have the same, but opposite problem. And if it were easy to change it, we'd all be ripped.
To answer the question
How to improve stamina without burning too much calories?
You're asking the wrong question. Instead of being worried about burning too much calories, you should be worried about how little you're eating.
You should be doing all the cardio you can muster, and then you should be eating enough to sustain yourself. That's all there is to it.
And again, I know it's not simple because you don't have the appetite to keep eating, but that's simply the problem you're going to have to deal with.
For fat people to lose weight, they have to refrain from eating when they're hungry.
For skinny people to gain weight, they have to eat when they're not hungry.
And if you want to change your weight, or stay where you are, that's the problem you have to deal with.