Ro'n i'n mynd can be either "I was going" or "I used to go".
Ro'n i'n arfer mynd is only "I used to go".
Arfer means "habit, custom" so it's like saying "I habitually went".
I put 'Yes, I was used to going there', but I'm not sure why it was wrong.
"I used to" and "I was used to" aren't the same thing in English.
"I used to" means "I habitually did" e.g. "I used to walk to school; I used to enjoy playing football; I used to go on holiday to the beach" etc.
"I was used to" means "I was familiar with" e.g. "I was used to the rain after living in Wales for three years; I was used to swimming long distances because I practised a lot; I was used to my husband's weird habits after a few years" etc.
Note after "used to" it's the infinitive: "I used to go/see/be" whereas after "be used to" it's the "-ing" form: "I'm used to going/seeing/being"
'Yes, I was in the habit of going there'? Rather roundabout, but 'used to' is usually contained in any imperfect, isn't it? Why would you need 'arfer'?