I have experience of this myself
I worked in electronics / software / IT sector and had worked for the company
for 7 years , I was on decent enough terms with my boss and an opportunity
came up for going self employed and I took it.
I handed in my resignation letter and had a chat with my boss and his manager
and told them I had always wanted to run my own business and that this
was a great opportunity for me.
I left the company and headed out on my own , after about 10 months I knew
things were no going as well as I needed it to be and I put out feelers to see
if I could get back .... they wouldn't take me even though they had
to hire 2 people to do the job I had done single handed
I found out later that my immediate boss had taken it thick that I wanted
to leave and thought it made him look bad, so even though an opportunity
for my old job had come up he blocked me from getting it.
Moral , made damn sure your decision is the right one , and try to be as
diplomatic about leaving with your current employer before leaving.
Sounds like you did everything right and your old boss was a jeb.