Hi Frances – You’re right, from a present day perspective, that many of the mutations don’t make obvious phonetic sense – although some of them, like @louis’ Ym Mangor, do.
The thing is, as far as we know they were all originally perfectly phonetically-plausible assimilations in origin; but languages sometimes seem to pick up on meaningless, incidental features, and evolve in a way that turns them into meaningful ones that no longer just happen automatically. (For example, English swim/swam/swum probably goes back to just a difference of accent in Indo-European between different forms of the word, depending on what ending followed it. All of those endings were lost centuries before the earliest Old English, but it doesn’t matter, because we can use the vowel to tell us what part of the verb we’re dealing with.)
So Celtic languages in general have tended to take those automatic assimilations and turn them into meaningful features, so that we’re still making the changed sound today, even though the sound that originally triggered the change – sometimes even the whole word that triggered the change – has long since vanished.
The easiest clear example that I know of is the word for ‘my’. The academic and historical Welsh dictionary, the GPC, gives a reconstructed pre-Welsh form men, more or less like English ‘mine’, but by the time we get to Middle Welsh it’s mostly written something like fyn (in Modern Welsh spelling). In the South I know some people do in fact say yn, but in the North and the written language the form you’ll come across is fy, with no ‘n’ on the end. But despite the disappearance of the ‘n’, people in the North and in written Welsh still do a nasal mutation on the front of the following word, if it starts with a sound that can be nasalised. In fact, that’s such a clear signal that you mean ‘my’ that you can even miss the bloomin’ word out altogether, and just do the mutation, and it’ll still sound like you mean ‘my’.
So you get a situation where ‘my brother’ in pre-Welsh Brittonic would have been something like men brōt- (mine brother!) but turns into:
- Middle Welsh vym mrawd
- Modern Welsh fy mrawd
…with no obvious rhyme or reason.
Now, because all these changes have become grammatically meaningful instead of phonetically automatic, you could just sit down and try to learn them all, and then try to remember when to use them when you’re speaking; and it would be dishearteningly hard work, but thoroughly traditional. But – and I say this as someone who literally read a book entitled “From Proto-Indeuropean to Primitive Germanic” for fun and owns a couple of different Hittite grammars – it’s probably much, much more effective to try to allow yourself to go with the flow (with whatever accommodations may suit you personally, as suggested by @aran) until you’ve heard fy mrawd so many times that it’s just what comes out of your mouth automatically when you need it.
And as others have said – dal ati (keep at it) and pob lwc (good luck)!
Please rest assured, I have not been discouraged, just not very good at ballroom dancing or the other things you mentioned so I couldn’t quite relate 