This NIH study says "The most common reasons cited were pressure from a parent (36%), transitioning was too hard (33%), too much harassment or discrimination (31%), and trouble getting a job (29%)."
This SEGM article also says accurate rates of detransition [and regrets] is unknown due to "inadequate follow-ups", "high rates of loss of follow up", "Imprecision of the measurement of detransition and regret", and "Reliance on biased samples or samples with poor generalizability".
Although the above are not specific to trans youth, however these articles 1 2 say that 97% of trans youth do not regret transitioning.
Overall, rates of regretting transition are highly exaggerated.