How do you know your in love?

My story is a lot different than most. And I mean this about love stories. I know I'm 15 and I shouldn't be thinking about love and all of that. But every guy that I've been with they say I love you so quickly and get mad or sad when I don't say it back so I say it back so I won't hurt their feelings. I don't mean not to mean it. I just don't mean it. It makes me feel like a bad person sometimes. My heart never flutters and I never feel nervous around anyone. Some told me I'm cold-hearted. I don't think I'm that bad. Maybe I'm just honest about my feelings. But I really want to know what it feels like to have someone you love and he or she loves you back.