Quote From bob86:
Hats off to anyone who has managed to complete a part-time PhD while holding down a full-time job. Truly, you have my utmost respect. Personally, however, I have to echo what AislingB says. I started my part-time engineering PhD in 2014 and realised around the 2 year mark that in order to have a realistic chance of ever completing it I had to quit my (engineering) job. So I did in 2016. However, the stress and feeling of isolation of those 2 initial years had such a profound impact on me that I’ve been suffering from depression and anxiety ever since. I think what contributed to my issues in particular was the fact that I went into the whole experience completely unprepared for it: I went straight from a (1st class) BEng into the PhD without first doing an MSc/MRes in the subject matter of my research. As a result, I have always felt like I’m playing catch up in the foundation knowledge department which has massively hindered my confidence. Despite these deficiencies, however, I have worked extremely hard and am now at the writing-up stage with 1 core chapter and introduction/conclusions still to write.

Although nothing can truly prepare you for a PhD, having a relevant masters with an element of research in my opinion can go a long way. Also, as AislingB points out it might be worth considering an EdD which is generally more structured than a PhD.

All the best!