But on Dec. 7, 2011, a month after Mark's death, Celebucki, who was then a moderator on GRACE, responded to one of her online threads in a long message that was atypical for him:
"There are times when I foolishly think that I'm the only person in the world that this has ever happened to; that even though I know that plenty of people lose cherished spouses, partners, parents, siblings and friends to lung cancer, somehow my situation is different -- no two people who loved each other as much as we did were ever torn apart so cruelly.
"But of course I don't have to go far to see how ridiculous that thought is. All I have to do is read posts from the many GRACE members who are so utterly devoted to their loved ones, and who feel such pain at their suffering. All I have to do is read your posts when you describe your love for Mark, and his for you."
Celebucki said he reached out as a friend with condolences, as he did with so many others, but this was different.
"Usually, I respond in short paragraphs or a few sentences," he said. "But for some reason, I saw Lisa's post and she seemed so devastated. I guess I kind of turned my thoughts inward and thought of how I was dealing with my own loss."
Things moved quickly from there. Within a few days, Celebucki offered his phone number and the pair talked for hours on end. It became a "nightly and daily" event. In the beginning there were "a lot of tears," she said, but they began to find it easier to talk to each other.
"I felt an instant connection," Sheehan said.
By the end of December 2011, as Sheehan's birthday approached, Celebucki told her he was "falling in love."
"I told him I was feeling all the same things," she said. "It was a whirlwind. I was very unprepared – feeling a lot of guilt and thinking, is this real?"
In January, Celebucki, living in Chicago, visited Sheehan for three hours in Dayton, then a couple of weeks later, he was coming every weekend. Renting hotel rooms got expensive, so near the end of January, a friend let them know about a house rental and by February 2012, they were using it every weekend.
"As mine and Jim's relationship turned from grief support to friends to so much more, I felt I was living a real-life fairy tale," Sheehan said. "A real Prince Charming had come my way. I went from being completely devastated and not knowing how I would get through the rest of my life to as happy as a person could ever be and feeling so blessed."
By mid-June, Sheehan was wearing a three-diamond ring -- "for the past, present and future" -- and Celebucki had moved in with her full-time.
"When we first considered a diamond ring, she was reluctant to remove her engagement and wedding rings from her marriage to Mark," Celebucki said. "So we agreed that those rings would remain on her left hand, while the new ring would take its place on her right hand. As far as I was concerned it didn't matter where she wore it, I was just thrilled and proud that she would want to wear it."
"Mark and Liz had a part in bringing us together," Sheehan said.
Today, their love flowers among the memories of their mutual pasts: photos of both Mark and Liz, as well as urns of their separate cremated remains.
"This is something I could never have anticipated," said Celebucki, who now works full-time for GRACE as a moderator. "What I like to say is that I can describe everything that happened, but I can't explain it."
For Sheehan and her husband, who raised three children, money was always tight. But now, Celebucki, who has no children, has enabled her to travel. "He took my daughter and I on our first plane ride ever," she said. "Jim and Liz had traveled a lot. One of the first things he told me is, 'I want to travel and take you places and show you things.'"
"I never imagined I would become a widow when I did or that my children would have to go through life without their father," she said of children who are 28, 23 and 14. "I also never imagined that after such a tragedy, life would become wonderful again."
Said Celebucki: "Everyone to whom we have told our story has been deeply moved by its message of hope in the face of extreme adversity, whether they are fighting a cancer battle or some other difficulty in their life.
"For that reason we would like to share it with as many people as possible, to show those without hope that it is possible to get a second chance at happiness."