this is a very common issue, it's just an extension of Shy Bladder / "stage-fright". (not being able to go somewhere besides in your own bathroom) Everyone will give you the same advice:
(1) don't hold it until you're desperate, don't think that drinking a ton will somehow help, it won't help much, won't be fun, and could cause injury
(2) continue to practice what works to get your body more used to using your diaper - wear a diaper that won't leak so you mentally trust it more
(3) keep changing up your circumstances a little at a time, work your way up to more challenging situations like in another room, while clothed, while sitting, while laying down, while walking, in public, etc. It should be natural to take baby steps with your diapers, right?
(4) everyone works at their own pace. some can get full command in days, others take months, don't stress over it if it takes awhile
One thing that could help is too start wetting while you're in the bathroom and then walk outside of the bathroom. This will help you to mentally get used to going outside. It will also help you to be able to go while walking.