The feelings a person might experience when lying in his grave is completely contingent upon the environment he has arranged and prepared with his deeds and acts in this world, thus this feeling may differ from person to person.
Considering the fact that once someone has passed away doing good deeds and any type of act, in general, is not possible, it seems that after being put in the grave one’s situation can’t change, although if there were to be a change in his situation it would only be possible through the things he has left behind whose good affects extend to after his death.
The hereafter is an unknown world, we know very little about it because those who have had the chance of witnessing it have not been given the opportunity to return and reveal its mysteries. Hence the only way to understand what death truly is, is by listening to what God and the leaders in which are trustworthy (the infallibles), have disclosed.
Regarding man’s condition after death and the feeling he has when he is being placed in his grave, the Holy Prophet (S.A.) has said:
The grave is either a garden of the gardens of Paradise or a chamber from the pits of hell.
Hadith similar to the above have been narrated from the Imams as well.
In another hadith that in a sense is similar to the one stated above, the two angels that address the dead person that has passed away concerning his deeds appear in different forms depending on the person who has died. If the person who has passed away is a righteous person the two angels that appear before him are called “Bashir” and “Mubashir”, these names themselves indicating glad tidings for that person. But if the person who is lying in the grave was a corrupt person the same two angels are called “Nakir” and “Munkar” which implies mysteriousness and not knowing what to expect.
This is why we read in our supplications: "ادْرَأْ عَنِّی مُنْکَراً وَ نَکِیراً وَ أَرِ عَیْنِی مُبَشِّراً وَ بَشِیرا"Oh God! Distance Nakir and Munkar away from me and show me “Bashir” and “Mubasher” instead!
Without doubt it is our own deeds that determine our situation and position in the grave.
The feeling one might experience entering his grave is a consequence of the environment he has prepared for himself beforehand and it is clear that those who are living in a garden of the gardens of Heaven and will be greeted by angels that bring glad tidings along with them as opposed to those who will reside in a pit of the pits of Hell being greeted by angels that remind them of Hell will not be sharing the same feelings.
Regarding the second part of your question, meaning the possibility of changing and modifying our position in the grave, what we must pay attention to is that: the position one has in the grave is a result of what he has done in this world till the day he passes away and due to the fact that his life has ended in this world he does not have the ability to enhance his position unless he has left behind beneficial and righteous things that have lasted even after his death. Only in this case is there hope for any change and enhancement of the afterlife.
In relation to this the Holy Prophet (S.A.) has said: Once a mumin has passed away nothing can help him except these three things:
1. Continuous Alms (Sadaqah Jariyah), which are inclusive of anything that remains and continues to benefit others; such as mosques, hospitals and schools.
2. Knowledge that others benefit from after one’s passing away.
3. A righteous child (which is a sign of how that parent raised him properly) that prays for his parents.
Indeed! If one has left behind such things, there is a strong chance he will be in an improved position in the grave and all of the torment awaiting him (in the case of there being any) will be converted into sheer blessings.
In closing, we will refer to another hadith from the Prophet where he says:
“One day Prophet Isa (pbuh) passed by the grave of a person who was being punished and tormented. The next year he passed by the same grave only to see that there was not a sign of torment. He appealed to God to enlighten him of the cause for this occurrence and through wahy (revelation) he was answered. God said: Oh Ruhallah! A child of the dead person has grown up and matured and has repaired a passage and has also become the guardian of an orphan, thus I have forgiven his sins because of the child’s deeds.”
Studying the following articles can also help you:
Questioning in the Grave, 3813 (website: 4283).
Reward and Punishment in the Grave, 5674 (website: 6146).