Getting and studying some texts on harmony, orchestration etc. is really necessary
but there are some online guides to orchestration. The northernsounds website has "Principle of Orchestration" by Rimsky-Korsakov online with comments. And here at YC in the masterclass forum are several useful articles by Qccowboy.
The problems with this composition could be fixed with some revisions - introducing new themes, transposing to new keys, adding new harmonies. But it is a lot easier to write four short pieces than one long piece so perhaps breaking it up would be a reasonable approach.
The theme in the opening section could form the basis of a rondo: that is, the A section for say an ABACA structure where B and C are new contrasting themes.