I tried veganism and felt terrible
That experience is real and worth taking seriously. It is also usually traceable to something specific: too few calories, no B12, low iron, or simply not yet knowing how to cook this way.
I didn't go into veganism half-hearted; I made a sincere attempt, and over weeks or months I felt steadily worse, with less energy, poorer focus and constant hunger. When I went back to eating animal products those problems cleared up quickly, and my own body's response is better evidence about what I need than any general claim that a plant diet works for everyone. Telling someone in that position that they must have done it wrong ignores what they actually lived through.
Feeling worse after a dietary change is common and has identifiable causes. The most frequent are eating too little, because plant foods are often less energy-dense and people replace rather than restructure meals; omitting B12 entirely; low iron or iodine intake; and a sharp increase in fibre, which takes some weeks to adjust to.
None of these are intrinsic to eating plants: they are features of a particular attempt. A poorly constructed vegan diet will make someone feel unwell, exactly as a poorly constructed omnivorous one will. The useful response to this objection is diagnostic rather than dismissive: what did the diet actually consist of, and was B12 being taken?
Treating a personal story or an isolated case as though it outweighed systematic evidence. Individual cases are real, but they do not establish general patterns.
How it applies here: A single personal experience is offered to settle a question that population-level evidence addresses directly. The experience is good evidence about that attempt and weak evidence about the diet in general.
Inferring that one thing caused another from the fact that they occurred together or one after the other. Correlation and sequence are not causation.
How it applies here: Attributes the symptoms to the absence of animal products rather than to the identifiable features of how the diet was assembled: most often insufficient calories or missing B12.