I baked bread, half wheat and half ap flour and used olive oil. It turned out tasting good but it falls apart. What did I do wrong?

I've found that of I leave the dough a little wetter than for all AP it's a little less crumbly when fresh. Bread flour should help--more protein/gluten formation. Never turns out as light, soft and bendable as grocery store bagged loaves though, at least not in my experience. Tasty, but not the same.

You can trust Boulangere's recipes!

I am reading "Ideas in Food," by Aki Kamozawa and H. Alexander Talbot. They say, talking about dough relaxation on p. 91: " products with too little gluten are very 'short.' They lack the structure to rise properly in the oven and the finished baked goods are very crumbly and fall apart easily."
Boulangere has a very good whole/AP flour bread, http://food52.com/recipes/14905_hearty_harvest_bread. She gives very detailed directions that got me baking successfully again. Maybe you can try this for technique (and a delicious bread), and apply it to the recipe you used.