Bethenny Frankel's Late Boyfriend Dennis Shields 'Had His Own Demons,' Source Says

While Dennis Shields’ death remains shrouded in mystery, it appears Bethenny Frankel‘s late on-off boyfriend may have been facing a private struggle.

Shields was found dead of a suspected overdose in his Trump Tower apartment on Aug. 10. He was 51.

On Wednesday, the New York City Medical Examiner’s Office announced that an autopsy was not performed on his body due to a “religious objection.” Thus, his cause of death has been ruled undetermined. (Frankel declined to comment via a rep.)

A source tells PEOPLE that the New York City-based banker’s death is “shocking,” adding: “Dennis was very quiet and reserved, but he had his own demons.”

Of speculation that Shields had a substance abuse problem, a second source tells PEOPLE, “I think it was something he was battling since his back surgery.”

A spokesperson for the Medical Examiner’s Office was unable to elaborate and told PEOPLE there would be no further comment.

“All we can say is that we did not complete the autopsy so we cannot definitively say what the cause of death is,” said the spokesperson.

Frankel, 47, first met Shields almost 30 years ago when he dated (and later married) one of her high school friends, Jill. Though she didn’t stay in touch with her classmate, Frankel and Shields shared other mutual pals and began dating in 2016, several months after he separated from his wife. (Jill previously addressed Shields’ death in an exclusive statement to PEOPLE, saying: “We are all heartbroken. Dennis was, and will always be, the love of my life. His spirit lives on in our children. Please respect our family’s privacy at this time.”)

A source previously told PEOPLE Frankel and Shields’ coupling was “unconventional and not always exclusive.”

“Bethenny loved him a great deal, but it was an unusual relationship,” said the source. “They would be together and then not. Sometimes these traditional relationships aren’t the best thing for everybody.”

RELATED VIDEO: Bethenny Frankel Says She ‘Never Knows Exactly’ Where Relationship Stands with Dennis Shields

Nevertheless, the source said Shields was a “steadying rock” for the Real Housewives of New York City star.

“He was very mild-mannered and she is like a force of nature,” the source said. “But she needed that in him.”

Despite the complexities of the relationship, a friend told PEOPLE that the couple considered marriage in the months before his death — and he even gave her an engagement ring, according a source.

On Sunday, Frankel spoke out about her sorrow on Twitter.

“It’s hard to breathe & I appreciate you giving me the space & support to try to do so,” she wrote. “It’s excruciating-sudden death is no closure & constant ?s & memories.”

She continued, “Our relationship is current so it’s painfully raw. Trying to stay healthy & move through it w tears & close friends. Xo.”