i usually give some excuses which are often REAL like 'my mom nagging me, sorry' 'gtg, need to catch some sleep, haven't been sleeping well' 'sorry, gtg got to study which I desperately need cos there gonna be a test' and they will often sympathise with you (which they always do) and will say call you next time or ttyl when free. and they will think that you really want to talk to them but you cant, ,so hinting to them that you want to talk to them again, but you actually want to end the convo asap.
Srry short. gotta go! ttyl bye that'll give you time think what would be a good reason for leaving suddenly. If you were trying to end the conversation for a jerk, you just disconnect and delete the message. If it's just a normal conversation with a friend, you should be able to say the same thing, or like, "Thanks but I gotta run. ttyl. bye. Make sure to stop texting then. Seriously, some people talk way too much, and some not enough. Never feel compelled to keep talking to someone when you'd rather not or when way enough has already been said or it's getting overly annoying to you. . You don't do you or they a favor that way.