Hey
Here is a link to an article I wrote all about the use of a process, although there is an emphasis on lyrics writing in the article it applies equally to music writing and arrangement:
http://songwriting.songstuff.com/article/process-of-writing-a-song/
Hopefully that gives you some ideas.
On the fact that you find yourself getting bogged down in experiments.... I think a good idea would be to adjust your music time and his you spend it. If you play an instrument you work best when you devote time to specific things. At a gross level a classically trained musician will devote time to scales and to learning new pieces, but as you develop you learn to be more focused. Yes you still learn new pieces and learn and practice new scales, but you will do chord practice, ear training, speed practice, accuracy practice, focus only on the hits you struggle with, performance practice, practice with other musicians, etc etc.
With songwriting and arrangement I would also suggest a similar approach. For speed of improvement it is vital that you focus time on what have problems with first and less time on the stuff you can do easily.
What I bet you don't do is have a dedicated "experiment" time for most of your skills. You will experiment while writing, but if you introduce a dedicated experiment time to your music time and try and focus payout experiments there, with the distinction that during writing a song you are focused on APPLYING what you have learnt.... You should make more progress.
Another useful idea... Have a dedicated notebook to write down lessons learned, what you tried and what worked and why, what didn't work and why... With a computer you also have the opportunity to keep indexed audio clips that your notebook can refer to. Fill it out as you go along and it is pretty easy to do and upkeep. Also handy ideas for recording and production techniques.
One other idea that falls under "experiment" is recreating other people's songs and arrangements as a pastiche. Just focus on the interesting bits instead of doing the whole song. It is a great way to boost arrangement and production skills and knowledge, leveraging off someone else's knowledge, especially when trying new genres etc.
The purpose of all this is to:
Compartmentalise tasks (including the task of integration of lessons learned!)
Improve the speed of learning
Consolidate and cement your learning
Improve recall and ability to apply your skills
I hope this helps.
Cheers
John