You’ve got two problems here - one is that the origin of the mirror is wrong, the other is that the direction is wrong.
The center of the mirror is the origin of the mesh - that little blob in the center of your circle. Select on of the vertices on the left of your mesh, where you want the mirror to attach, and press Shift S and click Cursor -> Selection. Go to Editing (F9) and click on Center Cursor in the Mesh panel.
The second problem, the angle, is even easier to fix. In Object Mode, press Ctrl+A and select Scale and Rotation to Obdata.