Honestly I think it depends on how much you drink now as to how you deal with this. If you are a heavy drinker, yeh you need to cut it out but don't do it cold turkey. One or two cups a day is fine (as long as they aren't massive cups).
As for breast feeding, it takes about 1hr for caffeine to get into breast milk, and about another hour for it to be removed (it doesn't sit or store in the breastmilk). So if you like your coffee and have baby that is fussy then you can fed bubs, then drink your coffee
Most babies don't react to caffeine, my first was sweet, my second is a bit more fussy so I try to feed him then have a drink.
As for artificial sweeteners, if you are pregnant yes you want to stay away from them. Very little research has been done on artificial sweeteners in regards to foetal development (ditto for children, basically if they are under 18 they should not drink the stuff because of the way the developing brain deals with things different to adult brain). If you are breastfeeding you are fine having a couple sodas a day because none (or negligible amounts) pass into breast milk. That's if you are like me and love your diet soda. Btw I'm a scientist so I looked at some of the actual research papers not just web search for info.