Markov State Model in Tikz

I am quite lost as I am very new to latex. Could anyone lend me a hand to draw something that looks like this please? In advance I appreciate your help guys....

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{cd}

\begin{document}

\begin{tikzcd}[
>=latex,arrow style=tikz,
row sep/custom/.initial=6em,column sep/custom/.initial=6em,
row sep=custom,column sep=custom,
every arrow/.append style={shift left},
/tikz/annot/.style={draw,rounded corners,inner sep=.3ex,minimum size=1.5em},
execute at end picture={
\draw[dashed,<-] (k+1.south) to[out=180,in=-80] +(-.7,+.5) node[above] {$C_i$};
\draw[dashed,<-] (k-1.north) to[out=  0,in=100] +(+.7,-.5) node[below] {$3N_i$};
\draw[dashed,<-] (k-3.south) to[out=180,in=-80] +(-.7,+.5) node[above] {$C_e$};
\draw[dashed,<-] (k+3.north) to[out=  0,in=100] +(+.7,-.5) node[below] {$3N_e$};
}
]
|[label={[annot]above left:X_1}]|
E_i(N_i)_3 \ar[r,"k_1"{name=k+1}] \ar[d,"k_{-4}"] &
|[label={[annot]above right:X_2},label={right:\mbox{inside}}]|
E_i(C_i)   \ar[l,"k_{-1}"{name=k-1}] \ar[d,"k_2"] \\
|[label={[annot]below left:Y_1}]|
E_e(N_e)_3 \ar[r,"k_{-3}"{name=k-3}] \ar[u,"k_4"] &
|[label={[annot]below right:Y_2},label={right:\mbox{outside}}]|
E_e(C_e)   \ar[l,"k_3"{name=k+3}] \ar[u,"k_{-2}"] \\
\end{tikzcd}

\end{document} 