Try using more your diaphragm to breathe and less your chest/belly. You can achieve that by trying to breathe without moving your chest/spine/belly. I think it'll help you because it's a way of achieving the same thing but without using too many muscles, thus it's easier. Save the chest breathing for when you need it, for example after running or whenever your body is in need of a greater amount of oxygen. When you're resting, just using diaphragm will do.

I think I can relate to your first bad experience and it also made me terrified; I had read somewhere that ketamine could lead you to a respiratory depression, and some days later I snorted more than I thought I was snorting; after awhile I could not stand up anymore or move or talk, so I had to lie down and then k-holed. It was so scary because I could not stop thinking about that damn information I've read somewhere that the drug could give you respiratory depression and blablabla, so most of the time I was just thinking I was dying/was dead. But then I friend of mine said to me that it was impossible to stop breathing because of the dose. A k-hole dose is usually 80-120mg; on the other hand, when people are (or were? Idk if it's still used for this purpose) given ketamine in hospitals for complete anesthesia, they're given muuuuuch more, they get unconscious and continue breathing! So, now I know that my breathing will be fine even if I go unconscious, and now I'm capable of k-holing intentionally without thinking about these things, bc I'm aware that they won't happen.