Praise be to Allah.
You have to try hard to give up this habit, and take your thumb out of your mouth every time you realize that you are sucking your thumb. Whatever happens because of forgetfulness will not affect your fast.
The Prophet (peace and blessings of Allah be upon him) said: “Allah has forgiven my ummah for what they do by mistake or out of forgetfulness, and what they are forced to do.”
For more, please see these answers: 1312 , 13619 , 12487 , and 82308 .
And Allah knows best.
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