Elisabeth Forsselius is believed to have been the daughter of the clerk Jacob Forssell (d. 1807) and Anna Magdalena Schylander (1778-1848).
She married the composer Johann Christian Friedrich Hæffner in 1787, divorced him in 1807 and remarried the opera singer Johan Jacob Fahlgren (1786-1848), who had previously been the student of her husband and their tenant, the following year.
In the rapports of Armfelt, who observed the board meetings as a representative of the Royal Swedish Academy of Arts, Elisabeth Forsselius was described as irresponsible and capricious - however, these sessions where known to be stormy, and if the judgement was correct, she was far from alone in being so.
She played Anna in Siri Brahe och Johan Gyllenstierna by Gustav III in 1788 and 1790 and the title role of Minna von Barnhelm by Lessing in 1793.
Among her other parts were Sidonie in Armide by Gluck in the season 1786–1787, Ismene in Elektra (by Hæffner) in 1787–1788, Delia in Les trois sultanes, a play by Favart in 1789–1790, Weakness in Alcides inträde i världen in 1793–1794 and Armide in Renaud (both also by Hæffner) in 1800–1801, and the marchioness in Griselda by Friedrich Halms in 1809–1810.