Derek Fisher -- Marriage Imploded In Last 30 Days

Just one month before Derek Fisher pulled the trigger on his marriage -- he was singing a completely different tune ... praising his wife as the love of his life ... that is, until something went terribly wrong.

Fisher filed divorce docs in L.A. Wednesday seeking to end his 10 year marriage to Candace -- but on February 19th, he was gushing about how much he LOVED his "beautiful wife."

"There are people in your life that are specifically designed, by God, to bring you closer to what your life is truly meant to be @mrsladyfish #myrib is that one person," Derek wrote on an Instagram post.

"She has changed the way I view life and what's most important to me. Life would not be the same without my queen. PERIOD!!!"

In the post, in which he was celebrating their decade-long marriage, he continued ... "Happy Anniversary honey! Our best lives are ahead of us! Let's GO!!! #itpouredrainonourweddingday #wetknot #westucktogether #myrib #blessedlife #beautifulwife."

So what happened between February 19th and March 18th that caused Derek to have such an extreme change of heart???