i keep having ideas when it's hard to write them down (shower etc.) and if i do go to write the idea down i usually forget most of the details. and if i deliberately put myself into a situation where i can easily write things down (usually by having my ideas document open or by using an audio recorder), then i can't have ideas. it feels like it's only possible for me to either have ideas or remember them, but not both.
that's true, ideas come most easily when they would be harder to write down. There are 2 strategies for the shower. one strategy, if you think the idea is really that good, just get out of the shower and go write it down. Or, if you don't want to do that, here are some memory tricks. pretend you are explaining the idea to someone else, and start explaining it in as many technical ways as you can. Pretend you have to explain it to them so that they can reproduce the idea exactly, but, you are not allowed to actually demonstrate it. For example, if it's a sentence, say "the first word is ___, the second word is ____," etc... Count the words. remind yourself how many words there are. Say all the odd numbered words. Say all the even numbered words. Think of the first letter of each word, and put all those letters together. try to come up with an alternate sentence using the same starting letters. Try to access the idea in as many different ways as you can. If it is a musical idea, start thinking of the notation of it, imagine you are explaining the musical notation to someone else without singing it. List the notes in order. Count the notes. List the rhythmic values in order. If you don't know musical notation in this way, hopefully you have some other conceptual notation system you use to think about notes and time. What tempo is it? what song has a similar tempo? how many notes are in the first beat? how many notes are in the second beat? etc etc etc. Keep trying to access the idea on as many mental avenues as you can, and this will increase the likelyhood that you will be able to retrieve it. but really the best option is to just get out of the shower. Most other places besides the shower, try to carry a recording device with you, the most common one being a smart phone. and if you have that, then much of these techniques will not be necessary