I have to agree with Darcy re changing from big to small chainring at the front.
Changing under load will most likely cause your chain to fall off. It's better too change the front one earlier and shift your rear gears harder to compensate. This is especially if a steep hill is coming up.

Regarding cadence, I am a slow one, only averaging around 70. Both DH and I have big leg muscles and whenever I spin very fast I start to bounce in the saddle and feel like I am going to fall off.

I think everyone is different and it depends on how you are built. If you are tall and lean, your muscles might be better going at a high cadence, but if you are built for strength, then a slower cadence is right for you. It does take a while to figure out what works for you especially as you lose weight and gain fitness.