Just wanted to chime in that my mother tends to do this as well. I've come to accept that she really is just trying to find a way to continue being useful in my life. She's not aware that it sounds like disapproval; it's more that she desperately does not want to be discarded now that I'm an adult independent from her. This could be true for your mom too (though I bet she's also modelling this behavior learned from her own parents).
Something that worked for my mom was to hear a friend with two daughters explicitly state that unless her children ask for her opinion, she doesn't give it. You could mirror this to your mom, e.g. "Look mom, I appreciate that you're trying to help, but in the future could you refrain from always giving your opinion? It makes me feel like I have to explain myself all the time." Also point out that you need to be treated with as much respect as a stranger (you know, you're from an alcoholic family when...) to whom she certainly would not constantly give her opinions.
You could even step it up a notch and reinforce it as a boundary (e.g. walking away when she insists on stating her opinion regardless). This is a valid boundary.
The other alternative is to just stop sharing with your mom (sucks, I know) and setting yourself up for these interactions. I do this to a certain extent with my own mom; I just don't tell her the details that I don't want to justify/defend. I don't ask her what she thinks about me traveling for a week; I inform her of when I'm going. This helps to focus her inevitable comments and skip the conversation about whether a trip is a good idea to begin with.
Hope this helps, elena.