I can't quite place my finger on it, but I just have this sense that there are too many. That last sentence for example - did I need the comma in the middle of it or would it have worked perfectly well without it? It's the practical use I am most concerned with, rather than any sense of grammatical correctness. Nobody cares about that anymore, this is 2016! I was probably taught it once, eons ago, and indeed a short refresher on the internet does suggest that before a "but" is an OK place to put a comma, but also that it does not necessarily follow that where a slight pause might exist in spoken english that this is the right place for a comma to be inserted. Unhelpful. It seems to be quite easy to spot where you definitely shouldn't whack a comma, but quite difficult to know where exactly one is actually needed and where you might get away without one. If I am ever in doubt, which is frequently, I seem to put one in. That last sentence there, would it have been OK, or even better, if it had read " If I am ever in doubt - which is frequently - I put one in." Or perhaps " If I am ever in doubt which is frequently, I put one in."
Ridiculous though it sounds, I quite often trawl through old posts removing commas that I feel are superfluous. You could argue I should quit while I'm vaguely ahead. English is not an easy language so in one sense I am doing quite well to even get words out and should not worry about the real nitty gritty. For instance I manage to avoid many of the usual written pitfalls such as their, there and they're, and also your and you're - the poor use of these latter two in particular I find impossibly annoying, far more than I ought to I'm sure. I've also seen his and he's used more-or-less interchangeably, and just recently no and know. I have recently caught myself simply writing the wrong word in a sentence, hear instead of here, but managed to go back and correct it before I hit send as it just didn't look
At this point it is probably best to quote Nigel Molesworth, that fabulous creation of Willians and Searle, as he does a run-through of his various teachers in "Down with Skool".
"They teach english e.g. migod you didn't ort to write a sentence like that molesworth."