Good on you Jess- learning the language of a community where you live is the best form of respect, and can change your relationship with said community totally. But don't get too bogged down with grammar- and aim for effective communication at this stage- mistakes and all.
Not sure about your course, but as a teacher of German, French and EFL, so much of the grammar that is taught is totally un-necessary for effective communication, often has easier but just as acceptable alternatives and mostly not used by the 'natives' either. For instance in French the subjunctive can be easily by-passed, and the future too (ex: 'je dois faire' is just as correct and effective as the archaic and complicated 'il faut que je fasse'- and 'je vais aller au cinéma' works just as well as 'j'irai au cinéma', etc, etc - for those who speak French). Just get on with it, we are all much too afraid of making mistakes ... and check on grammar as and when you feel it is stopping you from effective communication (at a later stage- polishing the grammar will be much easier anyhow). A language, and particularly Welsh I'd say, is like singing- you learn by singing, not reading the score... Bonne chance.
Thinking of it, learning the tai-chi form was the hardest thing I ever did- as I had to miss some key stages and catching up was almost impossible- I did in the end- but I actually burst into tears in the middle of a class once, with sheer frustration.