Mesh Current Method.(Current assignment)
Your problem = errors due non-systematic assignments. You have wrong signs. 

My suggestions


when possible, use the wanted currents as loop current variables
draw arrows for all  voltages in a loop, draw them from + to - assuming the loop currents positive
start collecting the voltage terms from an easy to remember point and along the current direction (in the example the starting point is + terminal of a voltage source)


An example: