That happened to me a few months back when school was about to start I suggest focusing on something else like art or video games or something of that sort that will take your mind off of it or go for a run get your heart rate up so when it goes back to normal you know it’s normal don’t focus on your breathing anxiety and ocd mixed really sucks anxiety gives you the thoughts and ocd makes you obsess over them it’s an annoying cycle that I’m gonna be honest only goes away if you face it. I personally am medicated and it helps sometimes but sometimes it doesn’t. I definitely suggest therapy, knowing you aren’t crazy helps as well. Another way to stop scaring yourself is when your heart is beating and you can’t help but think if it’s going to fast try singing it to the beat of staying alive while rocking your foot. Sounds weird but helps!