Most leaf lettuces would be hard to grow in this heat, but can be planted a bit later in the fall and do really great. Don't know what kind you planted, but Black Seeded Simpson usually gets bitter (that's what my grandparents always planted) so I don't care for it and it's an old standard lettuce. We fell in love several years ago with Red Sails, which is now considered an heirloom variety, but it has never gotten bitter for us. When it gets hot it gets a bit tough and the flavor is not as great, but in the spring and fall it's a nice lettuce to grow. They may have had a particular variety that tolerates the heat better. Would be nice if you could find out exactly what is was.