Hi OP. Yes, that referencing is incorrect, but please don't get too stressed out about it! Firstly, unless you're at a university that has a different system altogether that I'm unaware of, your first year doesn't count towards your degree classification at all, precisely because everyone makes mistakes like these and you need time to get used to university essay writing. Your lecturer knows that it's your first essay and is likely to be more lenient, and you can always improve and avoid making the same mistake again in your next essays.
I would encourage you to have another look at one of the many Harvard referencing guides online (as I'm pretty sure that is the referencing style that you were using). It looks like your full reference was perfect, however you should not put this in the main body of the text. Instead, your in-text citation would look something like (Hellmeier and Weidmann, 2019: 71–108). However, the page number needs to be the exact page on which you found the information that you included in your sentence.
At the end of your essay, you need to include your full references in alphabetised order under the heading 'Bibliography'. This doesn't count towards your word count, but in-text citations do. I imagine that including all of that information in your in-text citations really pushed up your word count and didn't give you much space to write the essay!
If you need more help, reach out to your personal tutor and keep checking your emails, as it is likely that your university will organise talks about citing and referencing specifically aimed at first-years. On your course Blackboard/whatever website your university uses, there will also be guides on how to cite sources.
Make sure you know how to cite sources by yourself, but the website 'Cite This For Me' can generate in-text and full citations for you.
If it makes you feel better, as a fourth-year, I noticed that I had completely left out two of the sources from my bibliography that I used in one of my recent essays. Everyone makes mistakes and it is likely that many first-years are in the same boat.