Prone flyes and prone rows or a rowing machine can help strenthen your shoulder girdle stabilizers and scapular retractors.

But my first thought is: is your bar too wide?

Second thought: is the bar too low, so your neck is going into too much extension?

Third: is something about the way you are positioned forcing you to lock your elbows later in a ride?

What did the shop tell you to watch for when they did your bike fit? Sometimes if the new bike is a dramatic change in position they can readjust things so it's a less dramatic change, and then gradually work you back into the aggressive position as you get stronger.