If you have some panadol or nurofen then take them to deal with the worst of the pain and make it a bit easier to express.
Get a towel wrung out in hot water or something similar and use it to heat the area before you try. Be mean to DD and reduce her food a bit for a day so she is more likely to feed from it!
And a cold pack after the feed.

Mastitis needs your GP for antibiotics. Persistent blocked ducts can be unblocked by ultrasound from a physiotherapist - but make sure you have one who is experienced in this.