It is always difficult to start with. As Alice said you've got to get past the mental barriers. Small things can help like sitting on the toilet while wetting and drinking plenty of fluid. The more you wet the easier it will become and eventually wetting your diaper will be just as natural as using a toilet. Over time you should also begin to be able to wet no matter what you're doing and it's then that in my opinion you can really start to feel like a baby as you're quite obviously starting to lose control.

Now this is something I keep going on about and I'm not sure if anyone else has experienced anything similar but now that I've got used to wetting (and messing) my diapers I've found that when I am in diapers I've got to the point that I sometimes don't notice that I am wetting. But when I'm wearing big girls panties (at work or when I'm out) I have complete control. It's almost as though I've trained myself to use my diapers automatically. I'm not quite at the stage of messing my diaper automatically yet but it does take noticeably less effort for me to mess a diaper than it does to mess in a toilet. Sorry for the tangent it's just an observation I've made over the past couple of years.

You do need to keep wearing them to become comfortable using them. Ideally you should probably wear them at some point every day for a while simply to get used to them. If you can get a weekend to yourself at some point try wearing them 24/7 for the weekend starting Friday night and ending Monday morning. After a weekend in diapers I suspect you'll have become a lot more used to wearing and wetting them.