This helped for me

1) the world does not revolve around you

2) people actually have lives

3) people get busy or just aren’t interested in replying

4) #they’re not obligated to reply

5) if you want them to reply to something meaningful and important you’re talking about, say; please get back to me as soon as you can, if it isn’t something meaningful and just general banter I’d drop it or bring it up next time you have a conversation (not that they ignored you but the previous convo)

6) talk to them about it face to face if it bothers you that much and get perspective

If I don’t reply back to someone and leave them on seen it’s usually because I think the conversation has finished and there’s no point continuing it, or I’m busy. It’s important to recognise these points asap so you don’t get hurt and you don’t hurt other people over it. Good luck.