Great job! You might ask your friend if he'd mind taking you to some country roads--if any are available--in your area. I think it's far easier to learn road riding away from busy traffic, especially if you're nervous.
As others have suggested, your emphasis on the white line makes me wonder if you'r paying too much attention to it. I tend to think you might be looking down just a bit to much, rather than out. Eden and Kalidurge gave some good advice on how to hold a straight, steady line. Also, be careful about actually riding on the white line, especially if it's rainy or humid out. The paint makes for a very slippery surface.
I would also suggest going to an empty parking lot or quiet road and practicing looking over your shoulder (for traffic or other riders) without wobbling your bike. This is an important skill to have and will help you be steadier and more predictable.
Another element of holding a steady line is a good "spin." Without going into great detail (because there are many threads on this topic), you want to be in a gear that allows you maintance a cadence of about 80-100 rpm range on a flat road, i.e., at a pace that is somewhere between mashing your pedals with too hard of a gear and bouncing on your saddle with too easy a gear. And you should aim to make complete circles with your pedal stroke. When you ride the next time with your friend, watch his pedal stroke and you'll get a sense of what I mean.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher