Your posture is stoop-ed with forward neck and what we call sway back. If you look at your body from the side, you can see how your body is kindof "swaying backwards". This makes the back of almost everything you wear a little too long. You can see it in the pants you're wearing. The back rise is too high.
When you button the jacket, it just looks like it's falling because it is. Fixing that is a little more complicated because of the stoop-ed posture. The back of the jacket may have to be shortened and squared from the top then raised upwards and lowered at the hem to compensate.
Fixing the gaping when opened is a separate operation entirely that involves squaring the fronts at the shoulder line. That will raise the gorge slightly, decrease the opening of the neckhole, and likely involve a collar recut.
This is a complicated diagnosis/fix even for me and I could be somewhat inaccurate in it. Please wait for Despos to agree, amend, or add to what I'm saying.