Gum problem. Have seen all these things!

Hello, I have had done oral prophylaxis two weeks ago due to inflammation and plague buildup with no bleeding. When I asked the dentist if it's something serious, he replied it isn't. My gums are pink but I have problem chewing food. I also have minor open bite, for which I ll go for orthodontic checkup. Also, my upper right second molar adult teeth is missing and my baby second upper molar teeth never fell. When I chew food, it feels like I have strain in my lower front teeth. I am really worried if this is a sign of any gum disease. I have seen all these things regarding periodontitis and it has literally shaken me. And do I have to go for extraction of baby molar tooth if I opt for ortho treatment? I am just 18! I don't want to lose my teeth so soon. :(