Ashley Hamilton -- who was married to SD from '93 to '94 -- tells TMZ he's heartbroken by his ex-wife's death ... and despite the fact they were only man and wife for about a year, he remembers her fondly as someone who was in his corner during a difficult chapter.

AH says, "Shannen was not only my wife, she was my guardian angel. Even though things don’t always work out the way we hope they will, she stood by my side during some of my darkest times. My love and admiration for her lasted far longer than our short-lived marriage."

He adds, "Watching her battle that horrific disease with such stoicism was inspiring to many, but even more so to me since I am a cancer survivor."

Ashely says he regrets never reaching out to Shannen during her cancer battle -- noting he wanted to give her space and peace ... although, he clearly feels he should've connected now -- telling us this is a reminder of how short life truly is.

He finishes with this ... "The world lost a free-spirited soul and my heart aches for her and for all those who loved her. May she rest in peace."

As we reported ... Shannen died over the weekend, and just a day before -- she settled her divorce with her most recent ex, Kurt Iswarienko ... this after a nasty public divorce.

She was 53.