I am going through a similar dilemma. I recently applied for a Ph.D. but because I only had a Bachelors Degree I was placed in the Master's program with the option to switch to the Ph.D. after 1 year (the program is in Canada), which has been the plan I've had with my advisor, whose lab I am working in as a student.

I am about half a year into my Masters, however, I no longer think I want to do the Ph.D. here. I'd like to finish the Masters here, but not the Ph.D. a Ph.D. can already be a very isolating experience (all of the writing, data analysis, experiment running), and I am 3000 miles away from home and don't know anyone here. I miss the support of my family and friends. I also feel like it's just time for a change. I have been in this field since I was an undergraduate volunteering in a lab, and just don't know if I feel passionate enough about it to do a Ph.D. on it anymore. I've also found a researcher in my home city doing work that piques my interest that will be accepting Ph.D. students the fall after I finish my masters.

I feel like I have made my decision in my mind, but am having a very hard time figuring out how to broach this subject with my advisor. I have been in this rather small field since my undergraduate years, so I feel like I'm not only disappointing my current advisor but my advisor from my past lab too, who knows my current advisor well.

Has anyone here had to have a conversation like this with an advisor before?