How should I use my upper body while defending?

[Long post]

I'm not sure what to do with my upper body while man marking. My subconscious reaction is to shove them when they get the ball. It's a bad habit that refs have told me to tone down (they basically said just make it a little less obvious).

I also don't really know what to do with my arms. I like to keep them at a 45 degree angle during set pieces. Both to show that I'm not fouling and also so I can use my shoulder to stop them from getting past me. But it's hard to find the balance between being too soft and too rough.

Sometimes I feel like I let forwards get past me too easily, or that I should have been more decisive when blocking a shot. Or not being proactive in the air. It's like I only commit 99% and have a mental block.

I've been told that I'm good at the hard stuff but struggle with the easy things. I never had any coaching as a defender, and don't know any good resources to research it myself, especially for small-sided games (6v6, 7v7 etc) that I struggle more with. I always stay behind the attacker, but I've been told that I should play less cautiously.

I'm very inconsistent, and sometimes I act like I'm scared of the ball or being physical. It's just a bad mentality that I've gotten stuck in, like I'm afraid of making a mistake.

I'd like to work on being more consistent, starting by improving my upper body movements and being more aggressive rather than backing off the forward.

Any advice or links to look at? Thanks for reading.