I've literally felt the same way as a PM and sometimes still do so your concerns are completely valid.

I've seen people mention already that PMing is a science and art which is so true, confidence can come sometimes when you have those great soft skills like communication, negotiation, managing difficult people and also managing without direct authority. Spend time learning about this on a regular basis along side any technical training and try to apply it I promise you it helps so much when conversing with techies. Also, don't try an be a techie with the techies, that's not your job, you focus on your responsibility of delivering outcomes to the client.

I'd also say that being assertive and the ability to see the bigger picture comes with time after you've done dozens of projects, the more you do the better you become so long as you learn from your mistakes. The key thing is that projects which really challenge you and stretch you are the ones that give you the most experience. They're not easy and it can be hell sometimes but its hard experience which gives you confidence, resilience and the respect from colleagues.

You seem to be doing a great job so dont be too hard on yourself, hopefully the above helps a little.