If you went to the doctor they did a chest x-ray, I'm sure checked blood oxygen levels and everything then I think you might be having panic attacks. I've had them a lot over the years. For me it can often feel like I'm breathing but I have the same feeling as holding my breath and not breathing. It is terrifying. I've gotten dizzy too and knew I was going to pass out - though I never have. I used to go to the hospital a lot because of them it took me a long time to wrap my head around the fact I wasn't dying and there wasn't anything wrong with my heart. I was really convinced I was having heart problems for a long time and didn't understand why doctors couldn't find anything. Especially because my pulse would sometimes get up to 180-200.
Something I've realized can help sometimes is to slow down your breathing and start taking slow rhythmic controlled breaths when it's happening. Not overly full ones like you're trying to get oxygen in you that isn't there. Just enough that it feels like a full deep breath even if your still feeling like you can't get air. Let it out, breathe in again, let it out like you're doing a breathing relaxation exercise. The hardest part for me doing this is that I feel like I want to gasp for air but it's important to stay with the deep rhythmic breathing and you might feel like you're doing the exact opposite of what your body needs. If you can do the breathing like that sometimes when you're not having a panic attack or otherwise just feel relaxed I think it's important because then when you do it having a panic attack then it's like you're body is like "oh that's right, usually when we're breathing like this everything is okay." Then it can create the state in you where you start to feel like you're breathing just fine and your body starts to get this message that there isn't anything wrong and it doesn't need to be telling you that you can't breathe.