You're not going to get harder shins kicking bags and pads. You only get it by kicking hard targets.
Read what I said again.
It's not a theory, it's proven science. Bases off your description it sounds your shins aren't well conditioned. I feel your pain though, in the past I've clashed shins in sparring and hurt myself. However, that was feedback that I needed to condition my shins, now my shins are a lot harder.
I've had 12 fights in the ring. How many have you had?
You should try conditioning your shins to get a feel for kicks without shin guards. Generating more power than your bones can handle is pointless, for you'll never be able to use it without injuring yourself, to a reasonable degree anyways.
I'm wondering if you even read what I post? The level that I currently fight at, they don't allow shinguards.