Starting with the death of her mother, whose sleeper-induced suicide never stopped haunting her, Susan Ivanova fell into the role of the survivor. She survived her brother's death during the Earth-Minbari War, her father's a decade later, and lost a number of friends along the way. She befriended few during her years on Babylon 5, undoubtedly protecting herself from further pain. Of course, this did not necessarily help...
"All love is unrequited, Stephen. All of it."
Walls surrounded Ivanova's heart through her years on Babylon 5; whenever they would start to crumble, some event would come along and promptly repair them.
The first traumatic event that we witness involving Ivanova is the death of her father, from whom she had grown estranged. She copes with it in the same way that she deals with pain throughout the series: by not dealing with it, burying herself in her work and ignoring her feelings.
Her romantic relationships on the Series were less than ideal, starting with Malcolm , the old flame who comes back into her life. He turns out to be a militant xenophobe who she realizes she never truly knew. The shock of his true nature certainly makes her weary of opening up to anyone else...
...and then her relationship with Talia Winters blossoms after Talia discovers that Susan was right about the sinister nature of PsiCorps. But just as Ivanova lets down her barriers and allows herself to grow close to her, it is discovered that Talia's the person on the station in whom the Corps implanted an artificial personality ("Divided Loyalties"). The personality takes over, killing the real one. As a part of the rebirth ceremony in "Ceremonies of Light and Dark," Susan admits that she believes she loved Talia, adding a new dimension to her grief.
Ivanova's bad experiences with relationships make her severely less inclined to risk her heart again when Marcus Cole decides to win it. Dodging his advances, she only complains of his annoying traits...and then he dies to save her, and his death impacts her terribly, as shown by her conversation with Stephen:
"You know, all my life I've had problems with relationships. You may have noticed. The ones that I loved always ended up hurting me or leaving me. And the ones who stayed, they had nothing inside, no depth. After a while, I just decided to forget about it. And here was Marcus. I knew he'd never hurt me, and I knew he'd never leave me, and I knew he loved me. I knew it. And I just didn't want to admit it. And he gave so much, and he wanted so little in return. He just wanted a kind word or a smile, and all I ever gave him in two years was grief. And it's because I think I saw what I wanted, and I was afraid."
According to rumors, in season 5, she would have taken Lyta's place and fallen for Byron, only to be hurt again. I'm actually glad that didn't happen; she has enough grief. Still, I would have liked to see how her character changed during the season, whether she learned to speak to her heart again as Lorien encouraged her.
At the end of the series, "Sleeping in Light," reveals that she remains in her role as survivor; she admits to Delenn that she is starting to resent the fact. Still, the series ends on a positive note for her (as well as everyone else :); she leaves her dull job on Earth to head the Rangers.
And her life is by no means over yet. There's always hope (one of the major themes of the series).
After all, as a friend said when I was lamenting Ivanova's state after "Sleeping in Light," survivors can certainly find happiness eventually if they manage to survive their grief...