How to stop getting mad at my boyfriend for being awful with his phone/texting?

So my boyfriend and I are long distance and have pretty much been together for 3 years now (broke up once in between) and to this day the only problem we ever have stems from how bad he is with his phone. Sometimes he'll take HOURS to respond to my texts (anywhere from 3-10) and I have to text him first a lot of the time. Sometimes he'll wait until 10:30pm to call me and then fall asleep within 20 minutes. He's not overly busy, we're both seniors in HS and he doesn't work or anything. I try my best to be understanding but it's SO irritating and sometimes I have a really hard time not making some snarky little comment or acting annoyed because I just don't get what's so hard. We broke up for this exact reason once and it's the only thing we ever disagree over. Any advice? 🙄