I don't think there should be a -d on 'didn't use to'. http://dictionary.cambridge.org/grammar/british-grammar/past/used-to says that both are used, but without is better. The difference between them is the emphasis. 'I didn't use to' generally means I didn't usually, but I might have possibly. e.g. "I didn't use to eat Japanese food' - It certainly wasn't something I usually or regularly ate. 'I never used to' is much stronger. I either never had the opportunity, or I made an effort to avoid it. 'I never used to eat Vegemite, it tasted disgusting! But after trying it accidentally once or twice, I now quite like it'. So - they have a very similar meaning, but 'never' has a stronger emphasis.

'understand how difference' isn't quite right. Perhaps 'understand the difference' or 'what the difference is'.