I do suggest you talk to a professional. BUT, in the mean time, until you gain success from that, try to find another security blanket you can use to substitute that.
By the way, if this girl really does love you, she'll be nicer and more supportive of you, and at least try to help you get rid of the habit. You should at least show an effort though. Talk to a doctor alone or with her if that make you more comfortable, but at least make a good effort, or else you can't really complain that she isn't accepting you.
well wear them while you are at work or something and take it off before you get home and if you are actually using them to like pee and poop in and you have no medical reason then you are screwed up and need help
I can't say that I don't blame your girlfriend for not wanting you to wear them. Diapers are generally for babies or the elderly. If your set in your ways and your not gonna change; then I suggest that you find someone who is comfortable with your diaper wearing. She won't be happy until you make that change.
Do you wear the diapers for a medical reason? If not, trust me, you should not be wearing them. You should diaper-train yourself since you are more than old enough to do that. Look, probably 99% (at least) of guys do not wear diapers, so you will have fierce competition for your lady if you decide to go against her wishes and continue to wear diapers at your age. I think that is an incentive enough for you to ditch them. If you want security, get a dog.