Why do I feel like this ??

I hate that I feel this way, but I get really upset when my boyfriend goes out and has fun without me. I don’t know why I feel this way.

I know it’s immature.

Yesterday for example, my family was having a cookout and swimming in the pool and his family had a cookout. So we decided that I would go to my families and he would go to his.

I was at my families house from noon - 3. I had a nice time.

He stayed at his families house from like noon -7.

When I was back at our house and texted him asking what he was doing , they were doing a bunch of fun stuff and I was home alone.

I guess I could have went , but they live 2 hours away.

So I felt lonely.

I don’t why in the world I feel like this.

Any advice ??