First things first, I appreciate your honesty and willingness to take responsibility for your mistake.
There is no denying that you have made a mistake and you have been sitting on it for a month. The most advisable course of action is to acknowledge it before the assignment gets marked, which will help you get ahead of the situation. In the perception of your lecturer, your admission of guilt will be interpreted in one of two contexts: in relation to the plagiarized sections (implicating 300 words / 10% of the assignments) or in relation to the plagiarizer (implicating your entire assignment). There is no telling which way his/her/their judgment will lean.
No matter what, my advice will be not to ask or propose how you think the assignment should get marked for the following reasons:
- It may appear as hubris on your part. Any marker who is aware that they have to examine a dishonest work will most likely NOT appreciate being instructed by the guilty party how to perform their duty and how many marks to allocate. If you attempt to influence your marker into giving you less penalty, you will seem like an entitled person. Therefore, you should allow your lecturer to determine how they want to deal with your assignment.
- You should consider yourself very fortunate if the lecturer merely penalizes your entire assignment (by assigning it a 0) and refrains from taking any further administrative action(s). If your lecturer marks very generously by penalizing you only for 10% of the plagiarized assignment, I think you should express your gratitude to your lecturer at the end of the semester and pledge to never repeat your mistake. The latter scenario is highly improbable, given that the assignments are randomly subjected to double-checks for quality control at the end of the semester in most of the prestigious institutions and the instructors are reluctant to incur the responsibility of erroneous judgments by showing undue leniency to the students.
- It is generally accepted that, within any academic system, voluntary acknowledgment of plagiarism can mitigate the severity of the penalty, but the assessor (in this case, the marker) retains the discretion to impose a harsher punishment. So, I think you should hope for the best and prepare for the worst.
- Sometimes, the markers are legally compelled to report cases of plagiarism to a higher authority. It will considerably help your case if you disclose first, admit your error, and save some time and effort for the marker. But it will inflict more damage than benefit if your admission is accompanied by an unwarranted request to grant you partial marks for the satisfactory sections of the assignments.
Having said that, when it comes to ChatGPT and other generative models, we should have a separate discussion. Most of my students who consulted ChatGPT typically regarded it as a semi-conscience being, a consultant who they thought searched the internet and formulated its own decision by gathering search knowledge. Most students thought that ChatGPT was a purely generative model that can be distinctly driven by students' individual prompts (the process they 'proudly' denoted as their own "creative directions") and therefore its outputs can be construed as the students' own thoughts. A lot of prestigious schools even embraced the students' use of ChatGPT, in its early stages, for helping them out with their assignments. But as it turned out that they were completely wrong and ChatGPT merely stitched some information (verbatim) from the internet in a fluent manner, which can NOT be interpreted as the user's own thought and will be regarded as plagiarism. If you belong to this misguided cluster, you should present your case to your lecturer accordingly and indicate that you had an impeccable record up to this point (I am hoping so judging by how ashamed you are for this), which should act in your favor.