Is the non-occurrence of the awaited matter a consequence of masturbating after swearing with the words, "By God, O Lord, if I do it again, then do not fulfill for me anything I was awaiting its occurrence"? Is expiation due for this oath? And what is the way to achieve what is desired?

The phrasing mentioned in the question is equivalent to swearing an oath by Allah Almighty, and this oath has no expiation, because it is not intended for obligation. However, one must seek forgiveness and repent from the act of masturbation and from supplicating against oneself with harm. If this supplication is followed by another supplication asking for good from Allah, then it is hoped that the first supplication will not be answered, because Allah does not answer His servants if they supplicate against themselves or their families in a state of annoyance and anger, for He knows that they do not intend that.