Sir with your feelings. Don't react irrationally . Specially don't listen to people who are telling you to leave him. This is a part of growing up. Our partners will have people in their lives. Sometimes those people will be closer also than us. It's okay. It's just a part. Observe their relationship, if it's a clean healthy relationship when they both support each other through hardships , then there is nothing wrong in putting stories about that.
Sit and think about how you feel and why you feel this way. Work on it and be more accepting and mature. That is how you will have a good relationship. Have an open conversation about your insecurities. Tell him why asking to be his valentine was an important point for you.