How can I overcome being nervous and scared about failing and not achieving my potential in the Military?

I'm 28 years old and just enlisted in the National Guard. I've been feeling great since the moment I took my oath and feel a great sense of pride. I want to serve our country and that's the main reason I enlisted. I make good money as a leasing agent, but I want to serve and also challenge myself. I want to gain more experience and provide a better life for my son. These past couple days though I've had some negative thoughts that relate to my age and fear of not accomplishing my goals.

When I was in high school I took JROTC for 3 years. I considered joining active-duty Army at one point, but life took me somewhere else. I became a dad at 19 and had a long term relationship through a good bit of my late teens and 20's. This year I thought to myself, "If I'm going to serve, it has to be now." I'm 28 so it's a now or never type of thing for me. And I desperately want to serve this great nation. I know I'm not super old, and my age isn't stopping me from serving. But I can't help but feel a bit bummed out thinking I could have joined at a younger age. A part of me though feels good knowing that I can share this excitement with my 8 year old.

Another thought that has come to me is the fear of not achieving and becoming all that I can be. When I started my first job I was super energetic and interested in learning everything. I would constantly ask my supervisors if I could learn x and y positions and they'd put me there and I grew a lot. Over the years, however, I've lost that bravery to ask for a promotion or to learn something new. I don't know why I've put myself in this shell. I want to break out and reach for more, I just get nervous and shy. I've been leasing apartments for the past 7 years when I feel like I could have been asking for a promotion. I'm just scared that I'll carry this mentality into the military and that's not what I want.

How can I break away from this and actually become how I used to be and even better? I want to serve the United States to the best of my ability and want to reach for the stars. I want to get promoted. I want to advance and I don't want to get lost in the shuffle. Sometimes I remember that God called me for great things and that comforts me, but I can't help but get nervous and scared of failing. How can I overcome this fear of failing?