Completely agree with you. There have been a few occasions where I have been speaking Welsh and the mutation has just come out without me thinking about it. It just feels like the correct thing to say (hard to explain). So yes, I think I’d agree with everyone else and say don’t worry about it and try not to over think it.
Pob lwc Colin!
As you’re from Cinderford, I just had to say hi (and “Ow bist?”) - I have connections with Coleford!
I still get some mutations wrong, but I think the tendency is for us to notice the occasional ones we get wrong and not realise just how many we get right without even thinking about them.
As the others have said - don’t let it worry you too much, it is a common dilemma and native speakers don’t seem to mind. In fact, I have come across native speakers who say they aren’t 100% confident they get them right!
It’s how mamiaith, mother tongue speakers learn it. Just what feels right. Welsh language learners in classic classes, Welsh language teachers, and people who have learned grammar know the rules. Lots of other people speak Welsh, ‘correctly’ without knowing the rules.
You could try this…
:D:D:D:D
Hello Colin
Sorry I made a mistake
The SSiW group meet in the Lime Tree, Chepstow the FOURTH Saturday of each month.
I, the owner of a tattoo, which may surprise some of you, think I would prefer to make the mistakes, or learn the rules. Yes, they are confusing, but after a while you do get to know how they work.
Then there is a different but related problem of remembering which words and situations CAUSE the mutations. That, for me, is the hard bit, and no tattoo will solve that problem for me.
Good job Welsh hasn’t had the equivalent of the German Rechschreibungsreform … yet. 
(spelling reform).
I was quite bewildered when I went to my first ‘Gloywi Iaith’ lesson last year (grammar for fluent speakers), the first ‘proper’ Welsh class I’d ever attended, to find out that other people who had learned the language had all sorts of mnemonics for remembering the mutations (“TCP, Blinking Good Disinfectant” and the like). I was actually quite shocked that they didn’t just know them - the thought of actually having to sit down and learn them was rather odd to me!
The first time it happened I genuinely though the speaker was having a stroke.
Normal conversation in Welsh, and then out of nowhere - baked beans, fish fingers, sausage rolls, chaps with coals, my old mother says you.are.it. - and then back into Welsh again
Wait.
So not actually fluent then? How does that work?
Fluent is getting stuff wrong but just ploughing on, not stopping every other word to do a mental lookup of a grammar table.
Isn’t it 
Sorry if I misled - these people were quite fluent in the way that I consider myself fluent, in the way you describe (making mistakes but ploughing on). But as this was a grammar class, so we had written exercises, and in those the aim was getting the treigladau right - which entailed some amount of recitation of mnemonics.
Shwmae Siaron,
Actually dwi’n dod o Coventry (I’m a Coventry boy) but have lived in the forest for some time now. Thank you for your advice.
Diolch yn fawr!
Colin
Shwmae Llanddinol,
This sounds like an excellent idea. I’d be very interested in coming along. Can you let me know any more - size of group, etc. Perhaps i can join you on the 4th Saturday in May?
Diolch yn fawr,
Hwyl am y tro!
You would be very welcome to join the group and anyone else you know. We really needs new members.
The group was started some years ago by someone from Gloucester and Chepstow was selected as the location because it was the nearest place in Wales. This person has since moved away. The group mainly consists of people who are/have been learning Welsh with Coleg Gwent. There is only one person using SSiW on a regular basis and he lives in Gloucester. Last month there were unfortunately only two of us, me and Nic from Gloucester, other people were unfortunately away. Usually there would be 5-6 of us and there has been as many as 12 but that was unusual.
This, what I’ve posted in another thread might be useful for you. Pay attention to things written under mutations table.
As written in that topic, the except is from the great poster Ylolfa published and is called “Welsh on the wall”. There are many useful things especially for beginners. It’s almost “must be” on the learner’s wall somewhere.
Hwyl!
Tatjana
I hope @elizabeth_jane won’t mind if I quote her mutation of the week. When I asked her at the pre-party meeting in the Black Dog, Caernarfon where she came from, she replied “Dw i’n dod o Felbourne”. Very impressive / Drawiadol Iawn. 
Glad you were impressed. I am not sure what the rules are for mutating Aussie place names. But it rolls off the tongue better to mutate.
I’m not sure of the rules either but my Taffia friends and colleagues living or working in Brussels would always say things like “Dw i’n byw ym Mrwsel” or “Dw i’n dod o Frwsel” so mutating foreign place names is common practice. 
Well that is good news!
Whoa!