How much organic matter is in your soil? Here muck soil is often drained to create great conditions for lettuce in a soil close to 30% organic matter.
This is just a stab in the dark because, to me, lettuce is a spring vegetable that gets bitter under strong sun. However, organic matter increases the amount of available water substantially in case it is short periods of drought that are making your lettuce bitter. Pinetree garden seeds does carry a decent summer lettuce which I grow just to have for sandwiches, but spring is the time of “salad days”.
I have heads of lettuce out there right now that I bought as starters about 2 months ago. The cool spring held them back a couple of weeks but it takes a very hard frost to kill lettuce.