Welcome relatively-new-Dom
I have been where you are. It is difficult. You are second guessing everything you do, trying to measure yourself up to an imagined yard-stick that you have never seen. The magic ingredient you are missing is, as you have pointed out, confidence, that is all; while there is no magic bullet to instantly gaining the confidence you need, rest assured with constant work and attention you can build it up to where it needs to be.
I know you are not attaching any fault to your previous partner for the situation but your first paragraph suggests it is on your mind as a factor. So, first of all, get your ex-girlfriend out of your head as to why, in part, you are feeling awkward. Focus on feeling grateful that you had the experiences you had and that it put you on this path. You need to take hold of the reasons that you feel this anxiety and own them. They are your reasons. When they are YOUR reasons YOU can do something about them.
In my world, being a Dominant is not living a life doing the things that Dominants do. For me it is not a list of activities that makes any of us a Dom. Fred can do all the things another Dom does and still not pull it off; on the other hand Fred can do things that we don't associate with dominance and yet come across as very dominant. For me, dominance is not in the things you do, but in the way that you do them, and, as a consequence of this, the critical aspects are your thought processes. Which brings us back to confidence.
You can read all of the books and blogs and internet fluff (there is a fair amount of that) as you like and it will not improve your confidence - confidence will only be improved by doing and is best tackled across all aspects of your life. To be sure, I am not advocating going out to tie people up without training, or flogging people you don't know without any learning in the bank. But I say go about your day pushing yourself to have confident human interactions where you display dominant traits. It takes practice and gumption to do this but it pays off in many ways. (I also advocate getting into your local 'scene' to meet likeminded people so you can realise that you are not that weird after all.) And I am definitely advocating rolling your sleeves up and getting knee-deep into your D/s dynamic to learn what it means to you. Learn how to hold the frame, how to handle feelings, brattiness, discipline, romance, learn how to boost your communication and how to keep her in a high state of arousal. So what if you say the wrong thing, handle an issue the wrong way or otherwise screw up in your D/s dynamic? If no-one gets hurt and you learn from your mistakes then I say go ahead and screw up. You will probably be faced with a partner that screws up from time to time too. We learn our most valuable lessons from our mistakes, and you will screw up. By doing you WILL improve your confidence.
Overthinking is not a dominant trait - so cut it out!
On a practical level I'd suggest a couple of tricks that can take the pressure off you during a scene.
1. Always have the scenes/meets in a place that you control. Ideally this would be at your place. This is your space and she is in a very real sense entering your frame/world there. You also have all of your gear close to hand and already feel comfortable. Do it when you know you will not be disturbed.
2. Plan, in a general sense, the scenes ahead of time. If you have a rough plan in your head then you will know, if you run dry, what you want to happen next. Don't be tempted to do too many different things in a scene. Like the musicians say, less is more
Often the beauty happens in the space between the notes.
3. Practice/force yourself to use your voice. This is a learned skill. It feels really awkward at first but trust me she will absolutely get off on it when you start to get into it. You can say the most mundane phrases and if delivered well will make her shiver. I'd suggest practicing a few lines that you will find useful ahead of time. Say them loud, with confidence and intent. Things like 'get on the bed', 'lift up your legs', 'get your arms above your head, and don't you dare move them', 'look at me', etc etc are easy lines to start with. You can repeat them as many times as you like moving her from position to position. The actual words matter far less than the fact that you are saying them and are in control of the situation. She will become disoriented and focus on your instructions, the moment and her submission.
4. I found, at the start, that using a blindfold on my partner took a huge weight of pressure off me. Something about the eyes of a partner adds to expectation and it was good to remove them (without mutilation!). Blindfolds are generally very sexy and the sensory deprivation adds to keeping your partner off balance in a scene - not knowing what will come next. Blindfolds can also have the advantage of making your partner less inhibited.
5. Part of the beauty of BDSM and kink in general is that it takes the focus off the orgasm. Accept that there may be orgasms and there may not: orgasms are no longer the point. The point is that you will have an incredible scene with your partner that she will not be able to stop thinking about for days. That is a gift my friend.
I hope this helps, Carraway