This is the Boob or Bust Fit Check helper. If you haven’t already done it, please check the The Band Test first.
Discomfort under your arm is often caused by issues with the width of the wires.
If you are finding you are filling the cup properly, but the wires are digging in under your arm then the best place to start is seeing whether the wires are correctly cupping your breast tissue under your arm.
With your side on shot, see if you can see space between the wires and your breast. This means the wires are too wide for you.
If you can see breast tissue under your arm past the wire, then the wires are too narrow for you.
Moulded and padded cups are more likely to hide these issues, than a purely soft cup.
Once you have worked out what your issue might be, then the group will be able to help with bra ideas. Please ensure you include the bra you have tried with the issue you have found in your post.