Sometimes it's others that let you down, sometimes it's the situation, and sometimes it's you. Either way, disappointment is hard to overcome.
Maybe it was a job opportunity lost or car break down, we mourn over these things. Quick to enter a state of sulking we let disappointments frame our decisions thereafter.
Whether it was our own fault or not under our control, there is no benefit of replaying the past. No purpose to regret and wondering how things could have gone differently. Move on and set yours eyes forward, not back.