Wilting in greens is triggered by temperature, pH, and salt content. To reduce wilting, you can cool the vegetables or shrimp, make the vegetables more acidic*, or decrease their salt content.
My suggestion would be to cool the shrimp with an ice water bath or cold running water. This is the most traditional approach for shrimp salad. Alternately, you could apply an acidic dressing before topping, but wait to salt it until the shrimp have cooled to 140F.
Alternately, wilting of vegetables adds a rather interesting variation in flavor or texture to a salad. This technique has been desirable and in vogue at various times.
Why does this all happen?
Plant cells wilt when cooking breaks apart the cellulose-based cell walls, and allows water to escape, causing them to soften. To quote On Food and Cooking (pp 282):
When the tissue reaches 140F/60C, the cell membranes are damaged, the
cells lose water and deflate, and the tissue as a whole goes from firm
and crisp to limp and flabby.
Acids will impede wilting, because the cell walls are held together with hemicellulose, and it is "not very soluble in an acid environment" (pp 282). Table salt is a problem, because "its sodium ions displace the calcium ions that cross-link and anchor the cement molecules in the fruits and vegetable walls, thus breaking the cross-links and helping dissolve the hemicelluloses" (pp 283). Calcium has the opposite affect, so if you can use hard water or add calcium salts, do so.
*Acidic solutions will reduce green colors(On Food and Cooking, pp 280-281), but preserve texture (282).
Best Answer
I think the key here is that you've been buying oversized lettuces. Bitterness of lettuce plants is not something under your control. When the lettuce matures (on the plant), it eventually starts flowering (called "bolting") as a way to perpetuate its genes. Once this sets in, the leaves turn bitter due to the production of certain chemicals, and acts as a defense mechanism against insects.
The process of bolting is irreversible and if it has commenced in the plant, it will continue on in the cut lettuce head (perhaps at a diminished rate). Big lettuce heads are invariably more mature than the smaller ones, which is why you experience a higher occurrence of them turning bitter. You could try the other suggestions here, but I cannot say for/against any of them as I haven't heard of any of them being used to control bitterness.
My advice would be to buy small to medium sized lettuces. Young leaves are tender and less likely to turn bitter. One might be tempted to buy the biggest lettuce on the shelf so as to "maximize" the leaves/dollar, but in the end, you have to decide whether the bitterness is really worth it.