The first thing you have to realize is that your coach is putting players into the game that put the TEAM in the best position to win. If a coach is putting you into the post, it's because you are the best fit for that role and gives the team the best chance to succeed. If you can learn to accept your role, I have seen players elevate their numbers and end up benefiting themselves and the team.
If you want to be a guard, you have to put in the work at practice. You have to outwork the guards on your team and gain your coach’s trust at practice, not games.