A couple of things to add. I'll put the clinical thing aside and assume that's not the problem.

I've been through a couple of depressive times. What I learned is that the urge to go on comes from positive reinforcement. When you took your first steps as a baby, you were giggling your head off. The more you walked, the better you felt. That's positive reinforcement.

Now and again you may encounter a hard task that you really don't want to take on. It looms over you and pushes you down. Inaction is your enemy. The more you don't act, the more it looms. The cure is to act. Always act, never react. Move forward. The only way to get positive reinforcement is to move forward. These first steps are the hardest. You have to gut it out and move forward enough to have positive things happen for you. There will also be negative things. Ignore them, or if you can't do that, find a way around them. Heal yourself. As you get stronger, you'll have plenty of positive reinforcement, which in common parlance is called "joy."

In this case, I think you should trust your cardiologist enough to follow their recommendations. Do they want you to be more active, to ride, work out, etc? With what limitations, if any? The suggestion to ride where you have cell phone coverage or to ride with others is a good idea, at least to start with.

Every year for the past 35 years, my wife and I go off into the mountains on a 10 day or so backpack or climbing trip, just the two of us. We will be out of cell phone range, we may not see other people for days, and we rely entirely on our own resources. We haven't died yet or been badly injured, but we know we could be. The positive reinforcement for doing this is so great that we will not stop until we can't do it anymore. We're the same on our tandem bike. We know the risks and we accept them. The price for not accepting them is, to us, to die the little death.