Does this grave oath differ from an ordinary oath, and what are the consequences after resuming the storage of qat?

Storing qat is not permissible due to the harms that result from it. Therefore, it is obligatory to abandon it and refrain from it. If you swore by solemn oaths to abandon it and then returned to it, you have committed a great sin, and you must repent and seek forgiveness. You are required to offer a single expiation for all the oaths you broke. As for what you mentioned regarding cursing and eternal damnation in Hellfire and the like, nothing of that is binding upon you except seeking forgiveness.