They aren't phrases. The words 'in' and 'of' have different meanings. 'In' in this case means 'part of'. 'Of' means 'relating to'. I have some great people in my life. The people are part of my life. There have been some strange coincidences in my life. They are part of what makes up my life. There are some facets of my life that I still don't understand. This relates to describing your life. That was the greatest achievement of my life. In this example, the distinctioni is not as clear. I can't think of an example of where they are interchangeable in natural educated speech.

they are both used interchangeably . i think "of my life" is more natural but the first one "in my life" is used too .