I know the feeling, I had never sewn in my life when I started to make bags, my wife taught me to sew. It will come in due time, every bag you make you will get better and understand more. Let’s deal with the most important things now.
The first problem you had was thickness. The edges should be skived if leather or vinyl to take away the thickness so that the edges will be thinner when turned to give you a more professional look. Your lining is not big enough and this could have happen because you didn’t sew along your seam allowance, I will need to see your pattern to determine. It could have also happen because of the way you sewed the linning in too. You should fit the lining before sewing, when you sew on a flatbed how you sew effects the fit of the lining. If you sew from the inside you get one fit if you sew from the outside you get another. Sew all the way through the seam, it looks like that seam at the top is not completely together or again it is a thickness problem. Once sewn you can clip the excess leather off, clip upward not down at an angle toward the top, be careful not to cut the stitch. The applique wasn’t clipped properly, look at the shorts in each lesson there is a button in the class to get to them. Next time you make this type of strap or handles sew on both edges, did you use a stay?
Your lining pocket looks good! Get closer to the edge with your topstitch. The reason being the top of your lining over time will separate from the leather above the stitch.To be honest you did a great job for your first bag, you should have seen mine 🙂
The only real problem you have with this was a ill fitting lining. In the class I state that your lining should be 1/16 smaller then the outside. That is a starting point, this measure will vary due to the thickness of the turned edge. Since each material is of a difference thickness, you will have to make a judgment call. Always make your lining bigger to fit then too small. Bigger you can take in, smaller you have to start over because of the stitch holes.
Have you joined the Facebook group yet? Please do, I can talk to you in real time and discuss what went wrong in more detail.