Nah, you are not stupid... It shows you were trusting and believed that the person who ought to have been loyal to you and/or had your best interests at heart took advantage and abused this (precious) trust.

Any such abuse we encounter (in particular (as) children who are especially vulnerable and ought to be protected by adults/parents) only serves to underline how the other party was/is insidious, manipulative and at times, plain machiavelian. The blame lies at their feet but yes, we tend to feel stupid for ever believing and/or continuing to trust them. But it is not stupidity... just a good, trusting, heart that has been taken advantage of.