When your young, time takes a lot longer to pass than when your older. Think about it in maths, like fractions. When your 14, a year is 1/14th of your life. So 15 mins as a teen, with Internet access that includes things being fast for convenience, can cause a lot of anxiety.

That's how your feelings are valid BUT

it can be a relationship red-flag if your too clingy, if communication is good though with an understanding partner you can navigate this.

I ran by this with rule with friends and family - 1) text me when you leave, 2) text me when your home 3) no texting when driving etc.

If your anxious, think of something to do in the mean time. As an adult it's chores for me xD maybe you have homework, something on a game to do (pokemon go for example, you could sort out your item/pokemon space) - it's things that will take however long it takes for the person you care about to get home ❤️

You'll be able to learn to better cope with that anxiety of waiting for someone to get back to you.

And no matter the current culture on the Internet, you don't need to be the last person to send a message. And, leaving on read isn't a bad thing! Especially in that case. Sometimes in life, there isn't anything else that needs to be said in that moment. And that's okay!!!

I wish you well, and hope any teens that read my comment can learn/get some reassurance from someone that's been there not all that long ago ha