I'd like to learn how to say
` I really love you.
나 너 정말로 좋아해(I really like you)
나 정말로 널 사랑해(I really love you. usually for opposite sex when we are adults. well, of course we can use btw girls also. girls say that often each other, dont we?)
<similar expression>
나 사실은 너 좋아해(말은 안 했지만, 표현은 안 했지만).(well..we don't express our affection for them sometimes and friends or bf or gf can get mad after all or they don't think we love them. then you can say, 'I (truely)really like you(I didn't say that or I didn't show that to you much)
` Please don't be mad.
화내지 마. 화내지 말아줘.
<similar expression>
Please don't be upset.
속상해하지마. 속상해하지 말아줘.
or we can say this with question like following.
do you know that I love(like) you, right(don't you)?
내가 너 사랑하는(좋아하는) 거 알지, 그렇지?
hope it could be helpful. :)