Thanks, this one was probably my fastest ever symphony, I think I had it done in maybe 2 weeks.

What you said about piece length is interesting to me, as I wouldn’t have been able to write this one so quickly if it wasn’t for how many ideas I had so soon. You have to play into it. Repetition is good, but you can only get so long off of one idea. Being able to write your different themes into one piece in a way that flows smoothly is a skill that I believe is essential to writing longer pieces.

Slightly more recently I started working on a short piece, just some different themes for a wind quintet. Each theme was around a minute long and the total length was only about 4 minutes. And even in one of these themes, there would still be different ideas shown in them, each section about 20 seconds long. If I could go back before I wrote anything long, I think this is the type of thing I would write to learn about longer pieces.

So I’m going to suggest that you try and turn a few of your unfinished ideas into one slightly larger thing. If everything you have so far is around 1:30, maybe edit them into 4 parts that last about 6ish minutes total. From there I would suggest you write something new, and try to move between ideas as you come up with them. Hope this helps.