I know what you're talking about.
Firstly, it's a good sign that you are aware of your faults and your weaknesses. It makes you able to direct your corrections and actually get better. Think of how it would be if you weren't aware the slightest of the fact that you blocked up and just sat back and did nothing about it.
On the other hand, don't let this bring you down. I get the feeling that you're patronizing yourself. Ask these questions: can I fly a plane? can I fly this plane? Dont answer with "no I can't because I mess the calculations up" or some other pisspoor excuse. Yes or No. I think you'll come to the fact that you CAN fly the plane and having it at that basic basic level will help. Don't ever think that you're not worth it, not capable, not fast enough or whatever.
The fact is that a few people along your career has thought you worthy and capable of flying comercial aircraft so if they believe it, why shouldn't you?
So what it comes down to: Dare to think like a winner but don't let it completely overpower your selfcritism, you need it to get better but not so much that it makes you feel bad. You can control this.
Once you start controling this, I think you'll notice that your stresslevels drop down a bit. The other tips given above are also very helpful, like: are you giving yourself enough time? If you're stressed out already at check in, your not giving yourself a chance. Show up an hour early and really get a solid insight into what you're going to do during the day. Do you know your procedures well enough to use them? Read up on your OM-B for callouts and company procedures. Not being sure of what you're doing really adds to the stress level.
I really hope it works out for you, I've had a similar problem throughout my career but I've learnt to combat it with said methods.
Regards/LnS