Hi Kmshna;
One exercise I used to do a lot to improve arranging, is write out a detailed analysis of songs for myself. I started with graph paper and drew in a line. for every instrument that might start and stop, more wave if the part got busier, larger if the part got louder etc. You can make up your own icons, for different ideas.. you soon create a vocabulary for yourself. Find other songs that have ideas you want to incorporate, I had to listen to a lot of ballads with drums to get the feel of what a good drummer can do..
I did this for every instrument.. Then used colored felt tip pens when motifs, melodies were repeated on each track..
I eventually created a singing bunch of symbols... Sometimes I would notate the melody.. But other times just teach them the melody.. Then I would mark it up with felt tip pens, to symbolize, soft whisper, crescendo the energy or melody up/down.. staccato notes, long held out notes little steps for several notes on a vowel to move etc. I too can get too busy with multiple interweaving of parts, I love to do it. But got complaints.. I think the brain can take up to 4 - 6 different melodic parts at once, then it becomes to confusing, and the listener loses interest. If the parts are closely related, more can happen,, and it can get very interweaving, if it goes back to be simpler too.. all of this depends of the style or music and your particular skills.. Yes, time and practice, and an instant playback medium.. you play it, and hear it back right away..
I quite often play pieces all nite when I go to bed.. So I hear them a lot of times before I fall asleep.. Also just play and repeat while sitting around listening.. I make up a mental list, and then fix the issues.. Something may sound great the first bunch of listenings but by the 40th,, just sound like some extra stuck in, or it serves no musical need.
I would often scour the internet for good midi arrangements, or even buy some. I would lay them up in Logic Pro's score Editor, to get a visual sense of what they had done.. Even though this piece didn't have much in common with my piece.. It was a great visual aid, the in the build up and resolution of musical density..
Eventually I took some online courses at Berklee music.. Great but expensive.. There are free courses on line. Coursera.org is one. there are others. These will not curtail your creativity, contrary to what self taught musicians might tell you.. They are tools. You then decide when to use that tool, or modify it to suit your needs. But the point is you know of it, and what it can add to your musical abilities..
Keep up the great work..