Please reason step by step, and put your final answer within \boxed{}.
Find all injective functions \( f: \mathbb{R} \rightarrow \mathbb{R} \) such that for all \( x \in \mathbb{R} \) and \( n \in \mathbb{N} \):

\[ 
\left|\sum_{i=1}^{n} i(f(x+i+1)-f(f(x+i)))\right| \leq 2019 
\]
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