Is there a way to improve my grade after graduation?

There's nothing you can do to change your grade on a completed degree. What you can do is do well in your job so that no one cares about your old grades, which it sounds like you've done. So congrats on that! The real issue here isn't your grades, it's that you're still feeling a lot of anxiety about your college experience. I'd suggest talking to a therapist about that. You can't change your grade, but you can better understand the feelings you're having about your college experience, and you can change how you think about those grades. I wish you all the best.

The grades in your first degree cease to matter as soon as you can establish some success in a subsequent career, as you seem to have done.

If it really bugs you that your degree class was lower than you think you were capable of, then by all means take an Open University degree to prove that you can do better. A close friend of mine who has never accepted his second class degree, 40 years ago (despite a subsequent PhD!), has now accumulated no fewer than three first class Open University degrees.

But don't expect employers to be particularly interested in your undergraduate degree class once you have a few years work experience under your belt.

I doubt that anything you can do now will change the past. But you can prepare for your future if you figure out what you want it to be.

The old grades will stand, but if your future includes study and another degree you can work there to improve your prospects.

Your past will be evaluated differently by different people and for different situations. No one can control that. But if you have an acceptable explanation you can move on, generally speaking. Your past isn't an immutable indicator of your future. But your path forward may be a bit twisty, depending on your goals.

Once you have a goal in mind, it would be good to talk to someone knowledgeable about it for advice in moving forward.

And note that most people evaluate recent work as more relevant than older. So, once you establish that you can succeed, you can move past the older experience.