1. If you know a person is not good at following thru on a commitment to do something, stop asking them to do things for or with you. DUH!
2. As for an already existing situation, you can keep stressing out over it, or you can let it go, and get on with your life.
3. Is the person lying or not? Maybe, but so what, get on with your life. On the other hand, maybe something came up, and they can't go thru with what they said they would do. Are you gonna keep stressing about it, or let it go?
4. Sometimes people will tell you what you want to hear, but later on, realize that they are not comfortable going thru with it, but don't want to upset you by a flat refusal, so they come up with excuses for not going thru with things. Again, you can stress about it, or let it go.
The best solution for all of these is to let it go, and to not expect commitments from people you know are not good at it.
Hope that clears things up.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau