If you have ever been told by your partner that he/she wants a break how did you feel?

I am considering telling my girlfriend that I want a break in a few weeks. We are both in high school and have been dating for 16 months. It’s not that she did anything wrong to me. Lately, I just haven’t felt attracted to her the same way I used to, and I feel like I’ve been feeling this way for a little too long. I’m not sure whether I want the break to end or not, it depends on what takes place during it. I’m just a bit nervous to do it because I’m very important to her and she’s even said to me that she hopes we get married in the future, while I really don’t want to marry a girl I met when I was in eighth grade and started dating as a freshman. I want to see other people and experience different feelings. One of our friends dumped her boyfriend of three years the other day in odd way, and my girlfriend told me she should’ve just told him that she wasn’t into him. That made me decide that my girlfriend thinks rationally and respects other people’s feelings and that maybe I should tell her I want a break. I just don’t want to make her feel hurt because she does mean a lot to me, just not in a way that I love her. Her sister also is very scary and definitely doesn’t like me, so I’m worried she’ll find a way to give me problems that really aren’t necessary. I was wondering if any of you have been in an experience where you’ve been in a relationship and told by your partner that they want to take a break, and how did you react to this?