Disappointment hits when reality does not meet expectations. Do not set your expectations too high if you do not want to get disappointed. I am fully aware of that, I have been for a while. For certain things, I try to be as fair as possible in terms of expectations. Have a reasonable goal for the amount of work you put. If you don't study hard, do not expect to get an A on your exam. If you don't work hard, do not expect a promotion at work. And so forth.
Having said that, if the people that you prioritized in your life, that you cared for and dedicated your life for, that you lost amazing opportunities so that they could advance, do not even care one bit, moreover making you a priority, then being disappointed comes so naturally, don't you think? Let's not make excuses for these people. We all have 24 hours in a day, no one gets more, nobody gets less. Everyone has a little bit of or more money, a roof or more over their heads and plenty of clothes to wear. None of these people that I am talking about are living in poverty, their primary needs are satisfied. It is about gratitude that they are lacking of. It is about priority that they do not have, it is about respect that is not shown, it is about love and care for the people that have loved you and given up so much for you, that are not poured out.
I do not get why some people do not understand that. I may not ever comprehend that pitiful behavior. Maybe these people simply do not care. Maybe there's something else. Whatever it is, I trust that disappointment is in order.