You can trying working on your thoughts, feelings/emotions, beliefs, etc. I don't know anything about you, such as how much you struggle with things like this and/or how hard it might be for you to work on things similar to this that negatively effect you.
But what can help immensely is therapy (with a good therapist) and writing down what bothers you--both thoughts and experiences--the emotions and feelings these things give you, and the thoughts that these things give you.
As you are probably aware, it's easy to get stuck in vicious cycles of negativity/depression/anxiety/etc. One negative experience comes along that causes anxiety, stress, depression, etc and then these feelings cause other negative thoughts which cause other negative feelings/thoughts, etc.
The good news is that, the opposite is also true. You can get into the same kind of cycle, only with good thoughts and feelings, happiness etc. It's harder. And it can take a long time to get into this kind of cycle, because the negative thoughts, beliefs, and negative reactions to situations are ingrained in you.
Basically you have to start small and slowly work on re-training yourself and getting new habits. New default, ingrained reactions to situations, to thoughts. As I've said writing can help with this. Therapy can help with this if you have a good therapist. And you can do a lot of good just with your head, by just thinking (and you will obviously have to do this regardless of whatever else you do).
I'll take the situation that you posted to give you an example of something that might help.
You have something that you need tell your manager. First, you have to walk to her office. You know you've already had awkward or uncomfortable experiences in the past involving what you're about to do.
Stop walking, and think about how you feel. What emotions do you have right now? What thoughts are going through your head. Are these thoughts making you feel better, or worse? Are they going to cause you to not succeed in hesitation, stuttering, etc? If so, attack these thoughts. Counter them with helpful thoughts.
I'm going to assume that you are very capable of going in and talking to your manager without hesitating (unless she asks you a question and you need to think before answering), or talking too quietly for her to hear. So tell yourself that. You can do it. You don't have to have any problems. Take some deep breaths and try to calm down. No matter how calm you feel, you can always be calmer. That's one thing I've learned.
You could also write certain thoughts, ideas, good reactions, etc down on a piece of paper or small notepad and carry it around with you all the time. You could have different pages in the notepad for different situations or different people.
This all may seem silly, stupid, or unhelpful. But that's just how it can seem in the beginning, when you have used them much or put much practice into becoming more aware of all that goes through your head and body. These kinds of things have helped me/are things I want to start doing. I've made a lot of progress I feel, but I still have a long way to go.
Hope this helps.
