Most hurtful is a deceitful act of lie especially if you have great deal of expectation from that person.
We all care for someone special in our life and we expect the same in return.
Most important to us in early life are parent and friends. Later part in life is our partner, children and friends.
If any of above lie to us which is intentional and hidden for a while and when it is exposed then that is the end of our expectations from that relation. The relation may continue but that will be excluded from our entrusted relationship's list.
But on other hand it hurts the most and as long as this resentment is there neither we will forget the person nor forgive the person.
If we forgive somehow, relation will start healing again and may enter into list of our entrusted group.
But there are couple things that needed to be addressed here apart from past life memories which are of least interest to me.
Firstly are we fair in our expectations and if we are then may be that lie was circumstantial and may have a reason behind it with a feeling that the truth may hurt us.
Regardless of any rationale lie is a lie and no one can let it go without repercussions.
If we let it go then it means we have extraordinary strength and courage to forgive the person and this virtue is a divine virtue.
If we can not let it go, it means our expectations have been shattered and causes a long lasting resentment in our mind.
Just imagine if one of your entrusted person lie to you and you never bothered about it and you continue your trusted relation as if nothing happened. That shows it that you never expected anything anyway.
Moreover none of relations in this world are based on 100% truth. But if you feel that it is not true then most likely no-one will be in your entrusted relationship's list at the end.
Either a complete truthful person could be the one in your list but unfortunately no-one of such caliber exist on this earth except God.
So if you are completely truthful and expect the same in return, then only choice you have is to make God as your friend.
It is a separate issue that God may accept your friendship without you being completely truthful . Because He has ability to forgive those who even attempt to friends with Him.
So all worldly relations are based on trust on each other and being human we are ought to make mistakes and we ought to hide it simply to avoid the embarrassment or not to hurt the feelings of the person. But if the person knows that his/her partner has the ability to forgive or let it go then there is no reason for the person to hide his/her mistakes.