Hi. I made a personal language back in high school, before I had taken linguistic classes in college or read your book. I’ve grown very attached to it but one thing that's holding me back from evolving my language is that I didn’t design it with a stress system in mind. I’ve been trying penultimate stress across the board, but I don’t find that satisfactory. Do you have any suggestions for implementing stress post-creation? Thanks!
You may not have designed a stress system, but I’ll bet there is one. Just go over your words and mark where you stress them. Analyze the system and see what you come up with. No need to try to force some new system on your language—especially if you don’t like how it sounds. Some stress systems (talking about natural languages here) just have to be memorized—like English and Russian. They’re too unpredictable to remember all the rules, unlike a language like Hawaiian. Your language is probably one like English and Russian, which is what we call lexical stress. No problem with that. Just means you have to mark where the stresses are rather than write a rule that says “Stress goes here or here based on x, y, and z”.
Another alternate is to not have stress. French is said to not have stress (I bet @christophoronomicon can talk more about this). Basically, too much stuff got lopped off the end of words in French, and it left it without word-level stress. There’s a lot more phrasal intonation, so you have to figure out what the sentence is to figure out where the stresses go, rather than looking at a word in isolation. This might also work for your language, but you won’t know until you look at a longer text and read it aloud, see what happens.
So yeah, no need to worry, and no need to impose a rigid stress system if you don’t want to. The phonology usually supports a rigid stress system (like Finnish, where you get a lot of initial heavy and superheavy syllables that help to bolster its word-initial stress system), so if yours isn’t sounding right, it’s probably because your phonology doesn’t support it, so no need to square-peg-round-hole it on there.