I'm sorry to hear about your heart attack but try not to lose hope.
I've had two heart attacks, three years apart and both happened on the bike. Each warranted a stent and I was back on the bike cycling in a couple of weeks after the first with no ill-effects but soon developed severe lack of breath at any increased gradient. As I had lost a third of one of the large heart muscles, I put the problem down to that. It was no fun panting and sweating behind my friends on a hill and hear them arguing who had dibs on my bikes after I popped off.
Again at seventy-five, it's difficult to determine whether it's increasing age, the side-effects of the various medicines one takes, or the damage to the heart which is responsible for the difficulty and that doesn't help.
Now just to cheer you up, about three weeks after my second stent was fitted and I wanted to see just how I was on hill climbs I went full-out on my usual run which includes some steep climbs. No problems at all and I scooted up with no difficulty and I realised that I'd forgotten how it was to be able to breath normally while cycling.
I do know how difficult it is to maintain a positive attitude and particularly so if like me, cycling has been your life, but it's important that you do so.
It may be worth noting that the advice given by cardiac consultants following heart attacks do seem to differ, and in my case I was told to resume my normal cycling activity as soon as possible.