Always Sad

I have branches, but no fruit, trunk or leaves. 

What am I?

Guess what comes next in the sequence:

1 2 3 4 5 ? 

The answer is not 6. What is the answer?

How many seconds are there in one year?

Think of a number

Step 1: Multiply it by 3

Step 2: Add 6

Step 3: Divide by 3

Step 4: Subtract the number chosen in step one from the answer in step 4.

What is the answer?

When you want to use me, you throw me away. 

When you're done using me, you bring me in. 

What am I?