Q: Is a thought that causes suffering definitely always a lie? It seems too good to be true sometimes.
A: I speak about truth and lies a little uniquely: All there is, is truth. So there is nothing else, and every so-called "lie" is just a small or partial truth, although some lies are ridiculously small truths, kind of like a lottery ticket. It is true that if you buy a lottery ticket, you could win. Unfortunately, it is a vanishingly small truth.
This is true of all dualities, i.e. light and dark, hot and cold, wet and dry. They are actually just different amounts of one thing, i.e. light, heat, and moisture. The opposite "duality" is just the concept we use to point to the relative absence of the one thing that actually exists.
I also say that the truth is what opens, softens, expands your heart, your awareness, and your sense of your own being, and also quiets the mind. Something that is less true has the opposite effect: it contracts you and causes your mind to become busier. There is more about this perspective on the truth in part two of my book, Living from the Heart, which you can download for free here: https://endless-satsang.com/free-spiritual-ebook
Suffering is the movement of a very small truth: the idea that something could be different than it is. So there is a lot of contraction in suffering. But suffering and all small truths do not harm you, and they do not last any longer than the thought that contains them. Once you stop suffering, your awareness and sense of being begins to expand and open. Opening is the default movement....it is what naturally happens when there is no limiting thought/truth contracting your awareness.
In a sense, suffering is like a mirage. A thought causes you to contract and suffer, but it is just a thought, which is always a small truth. If you inquire into the thought and see that it is not very true, that seeing is itself a bigger truth and so the seeing opens and expands your awareness and being. The suffering dissolves the same way a mirage dissolves the moment you get close to it.
There is a lot more on my site about suffering as listed here: https://endless-satsang.com/search?searchword=suffering&searchphrase=all
Not sure what you meant by it seeming too good to be true, but the realization that suffering is just an illusion does tend to make the contractions not last very long. It gets harder to care very much about small truths, and so they tend to drop away more quickly.
I hope this helps :)