I just found this under section 2:
"Work permits are granted for a maximum of 18 months (several stays are possible as long as their overall duration does not exceed a total of 18 months)."
It appears that a work permit extension under the Young Professionals programme beyond 18 months is not possible.
Other than marrying your Swiss boyfriend, I can think of the following approaches to obtain a new residence/ work permit:
1) Obtain a concubine (Konkubinat) residence permit (may not be available in all cantons).
2) Find an employer who wants to hire you as a highly-qualified employee and who is willing to jump through the hoops to obtain a non-EU permit.
A non-EU citizen can be considered as "highly qualified" with:
a. Masters degree
b. Necessary professional experience
c. Comprehensive language skills, further education and rewards are highly beneficial
d. Annual salary of at least CHF 100'000
See pdf pp. 11-12 at this link: https://www.vischer.com/fileadmin/us...tzerland_2.pdf
Another possibility might be for you to enroll in a Swiss university as a student.