When putting your child up for adoption please let the adopted parents know the honest details of why you are putting your child up for adoption. I would highly suggest writing a letter to your child from you explaining things to them. I think open adoption is the best for the child but the truth in a letter would be my second choice if open adoption is not on the table.
I have seen adoptees and adopted parents that have been told misinformation which is vital to the adoptee. Adoptees need a since of identity and they need to know they were not REJECTED. If the birth mother was young, unable to provide or raped these are details that adoptees deserve to know. Some details you can wait until they are age appropriate. Make sure you tell the adopted parents that you love your child and to tell them when they are old enough to understand. If adopted parents want the best for their adopted child they will do this for them. There is NO room for jealousy or lies in adoption. Do what is best for your child. Consider their feelings above yours.