The thing to consider is perhaps not whether it is difficult to manage selling on Fiverr but whether it is more or less difficult than doing a different type of part time job. Consider the possibility of having to work in the traditional student type part-time jobs of waitressing, cashiers, bar work etc.
The positives of Fiverr over these, I think, is that you can organize your own time to a certain extent. It is much easier to do Fiverr related work rather than coming back from college to then turn around within an hour and go serve drinks to drunk obnoxious people for 6 hours before getting home at 1am and then having a class starting at 9am. This is what many of your peers will be doing.
That said, Fiverr has its irritations too but at least you have the option to pause everything for a couple of days without consequence if you need to rather than being at the mercy of over demanding bosses who schedule you for all the unsocial hours that the full-time staff don’t want to do - turn them down and you likely have to find a new job!
You will learn a lot from Fiverr (or those other jobs), skills for dealing with people that studying simply can’t provide and these will stand to you in the future so stick with it and persevere because it will only benefit you in the medium to long term.