What BJ said!
I think what you are experiencing is the normal struggles of someone starting out. The more you get under your belt, the easier it will be for you, just learn from what’s happening. You should absolutely make a tillering gizmo! With the woods I commonly work with, I know at what thickness they start to bend and rough to approximately that thickness. Then I work the outer 2/3 of the limb first blending it in to the inner. The reason is that you can put a 2x4 on a tiller tree and it will bend if pulled hard enough, and most of that bend will be in the center. Since your staves will want to bend in the center naturally, it is good for me to encourage it to start bending in the outer 2/3 first. Don’t get overly frustrated, it will come and you will get there.