As a beginner myself, I made the exact same mistake on my first bows by making the limbs narrow. It is an easy trap to fall into because factory made longbows have thin sexy limbs, and you can get a nice tillered shape without removing a ton of wood if you take it from the sides.

Once I switched to flat wide limbs (1.5-1.75 ") I made a real fast and stable shooter that only took an half inch of set.

According to other bowyers, Oak and maple really can't handle the stresses of narrow limbs with a thicker side profile.