I personally divide Diamond as two different ranks.
D5 and D 4-1.
D5 usually consists of players happy with the rank, and a small portion of players who really want to go further.
The ones that are happy are less likely to improve because of the mindset ( If you're happy why change anything ).
That group is also split between boosted players, the ones who were aiming for D5, and players who tried really hard to improve and eventually became content with the fate of finding their true rank.
To get out of D5 just keep playing with the right mindset and will to improve.
Teamplay is strong there and if you get that down you're good.
Start thinking of the macro game more.
Knowing your matchup to the T is a big thing.
Be aware of their cd, know in which situation you can do enough damage and what damage they can do to you ( that is learned through trial and error mostly ).
It's all nice in theory like this but it takes time.
Just keep playing with the right mindset.