I thought of two other things that helped us with our timid Bingo. Maybe one or both might help you too....To this day Bingo has a hard time with new situations or even little changes in her regular routine. So maybe if you establish a comfortable routine & adhere to it religiously it might give your dog the security of knowing what to expect.
One trick Bingo loves doing is to go & get her food dish at supper time & take it to her eating place, set it down, then sit next to it and wait for us to fill it with food. She thrives on the praise she always gets for doing her trick; her big eyes light up & she gives tons of happy kisses. But if we are in a hurry & just grab a dish & set it down for her, Bingo looks upset and sometimes won't eat all of her food. So we have learned that her routine & her praise are both very important to Bingo's well being.
We also don't raise our voices around Bingo. Though she has come a VERY long way since I found her, there are still remnants of whatever happened to her before I found her. We discovered that Bingo's confidence is shattered if she hears yelling. Recently we had friends over watching a football game & we did yell a bit until we saw that Bingo was tucking her little stubby tail & cowering and looking fearfully around the room. We immediately reassured her and toned it down, but she remained nervous for quite a while afterward. So we learned another lesson from that. Again, I'm hoping some part of this might help you with your timid one. It's a real joy to watch them gain confidence & see their true personality emerge....good luck!