Reading, like anything, is easier the more you do it. I can read bass clef and play it on bass or guitar. I can read treble clef and play it on bass OK, but I've found I have more trouble reading treble clef and playing it on guitar now. Why? Because I don't do it very often. I'll sit down with treble clef and work out melodies on bass so holding the bass and reading treble clef is easier for me than holding a guitar in my hands and readting treble clef.
So, just do it until it becomes easier. Again, the key point is that you do NOT want to be thinking "the middle line is the open D string". Music notation is about the relative pitches (intervals) NOT about physical location. That D on the middle line can be played a lot of places on the neck. So, learn how that D relates to the line above it in the key you're in, and to the space below it. That'll make reading a LOT easier.
jte