Praise be to Allah.
We ask Allah, may He be exalted, to have mercy on your mother and to cause her to dwell in the highest part of al-Firdaws; we ask Him to enable you to bear your loss with patience and to make you steadfast in obedience to Him.
With regard to what you are doing, we think that you should refrain from that, for several reasons:
1.. Writing these messages and words renews your grief, and the scholars disallowed anything that may renew the Muslim’s grief, lest he gave up striving and be afflicted with despair and impatience. For this reason there is a difference of opinion regarding the ruling on women visiting graves, which is either prohibited in all cases, or it is prohibited for women to visit graves frequently. It was said that the reason for that is what could result from that of infractions of Islamic teachings due to the renewal of grief.
Ibn Qudaamah (may Allah have mercy on him) said:
Because women tend to lack patience and become distressed easily, and visiting graves will stir up their grief and renew the memories of the calamity that befell them, and there is no guarantee that this will not lead to women doing things that are not permissible, in contrast to men." (Al-Mughni 2/430).
2.. Writing these messages and letters is distracting you from that which is better for your mother, which is offering supplication for her, asking Allah to grant her mercy and forgiveness. If you stop writing these letters and messages, and you accept the reality of your mother’s death, and you strive hard in offering supplication for her when prostrating, and at the end of the night, that will be better for her than what you are doing.
Hence what we advise you to do is to stop doing this, and to offer a great deal of supplication for her. We ask Allah, may He be exalted, to have mercy on her and to forgive her.
And Allah knows best.