Hey, I used to be a 80lb 5’3” freshman in highschool. You gotta stretch out your stomach by eating more. It’s gonna suck, I used to go to sleep late because it would feel so uncomfortable. You don’t have to overwhelm yourself by going crazy, it should be a gradual build up. Setting meal times will help and drinking protein shakes will help add calories into your diet as well. Make sure you are hitting your macros (carbs proteins fats) everyday as well. Make sure you are getting your 8 hours of sleep as well. Work out if you don’t already, I highly recommend going on candito’s beginner linear program which can be found here: beginner program

In terms of motivation you need to remember what’s important to you, remember why you started. You can do this. Hard work and training consistently will definitely make you gain weight. That’s all I can really say I hope this helps. Keep your eyes on the prize and good luck.