You've failed!  Don't you understand how the aliens work.  Let me give you a couple tips.  

The Aliens will send in reinforcement after reinforcement in waves of attacks. Use this to your advantage. 

Once a wave is sent out, wait until it breaks up and attack the units at the front. This way you won't be overrun by greater numbers.

If it gets too tight, fall back and regroup. The base is well fortified and can provide adequate cover. Good luck.