Help with improving pronunciation? (stutter)

So, yeah. I know how I'm supposed to pronounce it, but the thing is I have a small little stutter. I've been speaking English well for the last decade or so, so my stutter doesn't come out as much in English. However, something about the tongue consonants in Spanish trips me up, especially when I'm nervous, like during the oral exam. Especially the r sound. Try as I might, I can't consistently roll my r's. I used to substitute it for a German r, but then I thought it doesn't sound very good, so I normally try for an r that sounds a little more like a d. That seems to have gone unnoticed, but it does trip me up a lot when I'm trying to say things. I wonder if anyone has any ideas?