Sounds like a bent main shaft....the shaft that goes through the main gear, swashplate and head that holds the main blades. You can try to straighten it but its cost is only $6 to replace.

These main shafts bend easily so buy a spare or two while your at it.

a test you can do is....

remove the main blades and disconnect the tail motor from the 4 in 1 (note the way the wiring is connected prior to removal to insure proper reconnection)

Carefully check/measure to be sure the paddles are equal distance from the center hub and that both are aligned horizontal to the swashplate, when turned/aligned both side to side and front to back.

Spin up the main motor and look at the top center of the main rotor head. If the main shaft is bent, you can detect the "wobble". If your heli is wobbling enough to almost fall over, this should be very obvious visually with this test.