Does a divorce take effect with the statement, "I have decided to separate," after I had previously told my wife that I would make a decision either to keep her or divorce her?

Your statement to your wife, "I have decided to separate," is considered a notification and not a divorce. Even if it were suitable for effecting a divorce, it would be a kinayah (ambiguous expression) by which divorce does not occur except with the intention [of divorce]. So, if you did not intend divorce, then it has not occurred.