Wikipedia describes

Regret is an intelligent (and/or emotional) dislike for personal past acts and behaviors. Regret is often felt when someone feels sadness, shame, or guilt after committing an action that the person later wishes that they had not done. Regret is distinct from guilt, which is a deeply emotional form of regret — one which may be difficult to comprehend in an objective or conceptual way. In this regard, the concept of regret is subordinate to guilt in terms of its "emotional power."

Being an adoptive parent in an open adoption you are going to eventually be witness to birthparent regret.

It is a normal response. Perhaps they were misinformed (even if you were told they were). Maybe things changed as time passed. The situations that were preventing them from feeling able to parent may no longer seem relevant. Maybe nothing has changed, they just are wishing that they had not made the decision they did.

This can be very difficult to face from the perspective of an adoptive parent. You are after all, someone who has gained something(the ability to parent) out of a seemingly now regrettable decision. Not what you may have gone into adoption expecting. You may not have ever contemplated such a turn around, so it can take you by surprise. The regrets we may expect on some level, but if it turns to the more extreme form of guilt and hostile feelings and blame result, it can become devastating for everyone.

A lot of adoptive parents actually find themselves as the target of angry guilt that might result from intense regret. Of course in the afore mentioned definition guilt is a deeper, more emotional form of regret, and it also may make it hard for the person feeling it to be objective. Suddenly the birthmother who seemed confident of her choice is now telling you that her life is in disarray because of YOU and adoption The adoption decision most likely has caused some stressful changes in a birthmother’s life, but is it really because of you? How do you respond?

Empathy can be a healthy response. Letting her know that you understand her regrets can be a positive. Trying to change her regrets however will not help. She is entitled to feel. Allowing regretful feelings that are being constructively shown and are not causing you or your child emotional harm can be cathartic. The best thing you can do is to let your child’s birthmother know that you remain supportive and positive despite intense feelings she might be going through.

This is not to say that you need to allow yourself to be blamed or made to feel responsible for those feelings. You may even feel regret yourself that situations or decisions prevented your child’s birthparents from raising him or her, but your desire to parent a child who was in need of an adoptive home does not make you responsible for their later regret. If you tried your best to do everything in an ethical way, you have done the most you were able. You do not need to accept hurtful reactions or blame. Taking on this type of response can eventually fracture your relationship with your child’s birthmother and negatively affect your child.

So what do you do if you have put your best foot forward and there is still a lot of hostility coming your way?

Letting everyone involved know that you are taking a step back to allow some calming perspective is fine. This does not mean closing an open adoption, but it might mean taking a break from contact till everyone is in a more objective place.