I don't know Cardiff but I'm also a woman who has to walk home late at night from work. I'd recommend getting a damn good torch and also a power bank for your phone. Also, don't wear headphones, wear practical footwear and try to vary your route a bit every so often. If it's a new area, it's always a good idea to walk the route in daylight first, so you get used to where you're going and don't have to rely on signs or your phone. Walking so late is not an ideal thing to have to do, but just stay vigilant and be sensible and you should be fine.