Scheduling an appointment far ahead means you move stuff around it, not to see if it fits in your schedule.

Y'know that small-ish meme about dentists asking you if you're available at an absurdly far away date? Yeah, this is about that. This also extends to other things too.

If you make an appointment, an important appointment, be it seeing a friend or going to the doctor, you do not approach it as if it can fall into place. You have now have to plan around it. That appointed day should be set in stone, immovable. When other thing pop up, unless they are more important, (eg ur mom dies when you were supposed to go watch a movie) you need to be making active effort to meet the schedule.

So when you say "I don't know what I'll be doing in 3 months!" yes you do. On that day, you're going to that appointment. if anything else needs to happen, plan around that day already being taken