I started instructing part time, and loved it. Then for various reasons I switched to full time. I was dividing myself between two airfields, with long days, and a lot of travelling time. I was getting knackered by mid-summer, and the day I woke up on a day off and thought, "Thank god I don't have to fly today", was the day I realised something was going wrong. Then in the winter there was bad weather and no work, which equalled no money.
I'm now back to part time. I love it again. I don't get so tired, I don't panic about paying the bills when the weather's bad, and I remember who my students are without having to get in early to read their notes.
I think I'll keep doing part time instructing for as long as I can pass medicals. But for me, fulltime meant that I turned my hobby into my job....and it became just another job.