All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, you should know that this fear that you have is evidence of faith and fear of Allaah, so you should not worry.
You should endeavor to repel any evil thoughts that you may have in your heart, because this is one way that the devil mocks people and plots against them, as he is their enemy. So you should try to anger him by seeking refuge in Allaah from his evil. For guidelines in this respect, please refer to Fatwa 90226.
The punishment of those mentioned is not necessarily the severest in the absolute sense. Al-Aloosi said in his Tafseer that what is meant by it is eternity in the Fire and that it is severe insofar as it does not cease; or it means that it is the severest kind of punishment in comparison to the punishment of those who did not commit such disobedience, as the disobedience of the former is worse than the disobedience of the latter. At-Tabari reported another interpretation in his Tafseer, namely that they will be brought to a punishment that is more severe than the punishment of this world.
It should be pointed out that the acts of Allaah with respect to His creations alternate between Justice and Grace: if they are punished, it is from His Justice, and if they are in bliss, it is from His Grace, as He is the Generous, the Kind and He does not wrong anyone. Allaah has made injustice forbidden to Himself and He has made it forbidden to His slaves. Allaah says (what means):
{And your Lord does injustice to no one.} [Quran 18:49]
{And Allaah wants no injustice for [His] servants.} [Quran 40:31]
{…and injustice will not be done to them, [even] as much as a thread [inside the date seed].} [Quran 17:71]
Therefore, a believer should have good thoughts about Allaah and beware of having evil assumptions about Him, as this may lead to his destruction. Allaah says (what means): {And [that] He may punish the hypocrite men and hypocrite women, and the polytheist men and polytheist women – those who assume about Allaah an assumption of evil nature. Upon them is a misfortune of evil nature; and Allaah has become angry with them and has cursed them and prepared for them Hell, and evil it is as a destination.} [Quran 48:6]
Allaah Knows best.