At only about a year old she lost her father, after which her mother married Conte Barbon, Commander of the Castle of Milan. Once grown up she had success as a singer and dancer in the Italian Opera. However, possibly to escape the restraint of an unwanted stepfather, she married Giacomo, son and heir of the rich Fagnanis, among the most ancient of the Milanese families. 

Giacomo Fagnani had led a dissipated bachelor life and his parents were none too pleased to see him married to a reckless young woman. Not feeling welcome in her new family, they travelled from town to town where she amused herself with various lovers met on their travels while Giacomo indulged in his own amours. He left her temporarily to travel to Corsica where he met Paoli, the Corsican 
patriot. Re-united, they met the 10th Earl of Pembroke who was partial to Italian adventuresses. Travelling with both husband and wife, this did not prevent the Earl from making love to the wife. Constanza expressed her opinions to Laurence Sterne, a sympathetic listener, maintaining that life was too short to stand on ceremony. 

In the winter of 1769 the Fagnanis, together with Lord Pembroke, arrived in London. However, having lost interest in Costanza, Lord Pembroke passed her on to his fellow rake, the Earl of 
March, the future 4th Duke of Queensberry. In the meantime Giacomo, turning a blind eye towards his wife's behaviour, had wasted their money by gambling but was rescued by the Earl of March. 

On 25 August 1771, being at White's Club in St.James's, the Earl of March sent a note to his friend, George Selwyn, telling him that the evening before Madame Fagnani had been brought to bed of a 
girl, Maria Emily. The infant was handed over to George Selwyn, a bachelor, whose interest in the children of his friends had grown into a byword. Selwyn and the baby "Mie-Mie" became inseparable. 

After about six years, Costanza, to satisfy the grandparents who could not understand that the girl was left abroad with people not of her own kin, half-heartedly demanded the return of her daughter. 
However, Lord March refused to intercede for Selwyn who then was forced to hand over "Mie-Mie" in Paris. Costanza and Maria Emily then travelled towards Milan by easy stages. After a year of agony, George Selwyn went to Italy where, in the meantime, Costanza had given birth to another daughter. George Selwyn persuaded Costanza to return "Mie-Mie" to him and Costanza, now having another daughter, listened to him but the monetary considerations made her agree. George Selwyn promised to make "Mie-Mie" his heir. 

Giacomo, as a result of his dissolute life, had gone both blind and mad and, several years before he died in 1785, had been sequestered in a country retreat. After losing her husband, Costanza 
embarked upon a succession of none-too-affluent lovers; then on 24 January 1805, she had a paralytic stroke and died. 

Source: Leo van de Pas