I am basically an electronics tech, not directly IT but there are similarities. There are a few things I try to remind myself:
-In my field, it is much hard to train a new person, there are certain things that require on the job experience, it's not like my boss is out to fire me when he knows he would have to train a new person to do things I've done for years
-When I'm having a bad day, I make a To Do list at work, I write down what I need to do and also give it priority High or Low
-I work through each High priority thing logically: when do I have to complete this task? Is there anything I need (tools, materials, etc) that I need, that I don't have? Can any of my coworkers help me with this task?
-If there are issues or I think it's going to take longer than expected, I tell my boss. Calmly, not defensivly. I explain why I think it's taking this long.
-I remind myself that anyone can get fired or laid off. It's usually because business is slow. That is beyond my control. Unless I act like a total jerk or am completely lazy, I probably won't get fired. If business is slow, then tough, if I'm not longer needed I'm no longer needed and I look for another job.
I hope this help. Best of luck to you!