My question is: Is it possible to dive in cold water and actually not be cold? Or should I just restrict my diving in future to warm waters?
Yes, you can dive in cold water and not be cold.
It is your decision, if you decide to
restrict your diving to warm water.
You are a small person probably with a low % of body fat. ( Good for you )
Smaller people get cold easier. Fat people stay warmer.
Have you ever seen a skinny walrus?
For me water below 80F is cold water. At 78F, the water is 20F colder than your body. As you know you lose heat 25 times faster in water.
If I were you, I would get a Semi-Dry Scubapro Nova Scotia.
This suit is cut to fit people in good shape like you.
You would be smart to dive this suit in water from 65F - 80F.
You could dive it in colder water on shorter dives.
Then if you want to commit to really cold water, get a drysuit. The EverDry 4 is an option.
I will be diving in 93F this weekend, no suit needed.