It seems pretty simple. She was somewhere around 24 when Duncan met her. She had just graduated from a famous French art school and was making a living as an English speaking tour guide in Paris. Once Duncan started dating her Tessa never had to work another day in her life. She was swept up by a rich, handsome, generous, devoted man who kept her in a lavish lifestyle of travel and designer clothes. She was able to devote all of her free time to her art. From the outside she had it all, beauty, money, an intensely devoted lover/partner of 12 years and all the time in the world to be an artist.
Of course underneath it all were trials and troubles. For one thing she was plagued by the idea that while she was continuing to age, her immortal lover remained the same. Someday she would be dead and gone and the man she loved so much would move on to a new, beautiful young woman. This planted a seed of jealousy in her that tortured her unnecessarily whenever a female immortal passed through their lives.
And then of course there were those pesky evil immortals who saw Tessa as a pawn to be used in the "Game" with Duncan. In the 4th episode of the second season, she was kidnapped by one such immortal. At one point she escaped but when she ran into the street her kidnapper simply dragged her back into the house. It struck me as odd that an immortal skilled martial artist who was trained to kills for hundreds of years, never bothered to teach his lover of 12 years how to defend herself. One quick karate move would have saved Tessa's freedom and her life. But instead Tessa lived a few terrifying days in captivity before finally being found and freed by Duncan. And then in a terrible twist of fate she was murdered by a drugged out mugger as she was leaving the home in which she had been a prisoner.
The question is, would she have been better off having never met Duncan? She lived a short life (roughly 36 years) filled with love, romance, wealth and unencumbered (and well nourished!) artistic expression. But she died early. Anyone who saw the very last episode ever of Highlander got to see what her life would have been like without Duncan. It was a longer one, with marriage and children. But it was empty and lacked the richness that comes from expressing oneself and sharing their greatest gifts with the world and being nourished by a devoted lover. But...she lived. She was never kidnapped and never murdered.
One has to wonder which life was better.