How do i stop being sad all the time? (long distance relationship question)?

I'm 17, biual and in a long distance relationship with a girl. We've seen eachother twice in a period of 4 days each. We've been dating for 5 1/2 months. I know it's a stupid question, but I want to know if any of you in a long distance relationship are always or constantly sad. I should look back at our time together and be happy and look forward to seeing eachother again... yet I think about her being at my house (kissing, holding hands, etc.) and i cry hysterically. I wake up thinking she'll be next to me but she's not. We fall asleep on the phone together, but it's not the same as her being here. It's really hard for me. I know she's worth it... I just hate being sad because I've finally found the right person and everything is perfect when I'm with her...I should be happy.