Dress regret??

It's difficult to say without seeing the actual dress, but based on silhouette, I would think a mermaid style would be far more 'weddingy' than an a-line. And while it's great that you are happy with your body shape and size, you don't need to wear a fitted gown to prove it. (And I actually think the shop assistant's comment was quite inappropriate - no one is obliged to show their body off in a fitted gown whether they have curves or not!) You say that the gown is beautiful and suits you - I wonder if 'a-line' has become aligned in your mind with your uncomfortable, disliked dress of 15 years ago. It might help if you try reminding yourself that there are dozens of different types of a-line dresses, and the one you have now is nothing like the one you wore back then.

Shops don't usually take back dresses, but they will sometimes allow you to exchange, so if you are really unhappy with your dress, and they had others there that you liked, then it might be worth asking if you can swap.

Alternatively, you could try selling it - you won't recoup all your money, but it would be better than losing the lot.

But I would spend some time first considering whether you are allowing an 'a-line prejudice' to put you off a lovely dress!

When you say 'too big' do you mean that it is a size or two too big? Or that the skirt is very big. You can sometimes slim down a skirt by taking out some of the underlayers. It won't make it fitted, but it will make it less puffy and more flowy.

Can you post a photo of it (without identifying yourself of course!) Maybe having some random opinions might help?

It's a beautiful dress and really suits you, and I think you will feel better about it once it is altered to fit you - don't forget there is a lot of spare material pooling around you in that picture, much of which would be removed when it was shortened to fit you. You could also consider having the train cut off if you preferred the simpler look, since I think it's probably that rather than the a-line which is making you feel it is too big and 'weddingy'. It would be possible to have a little of the fullness taken out of the skirt if you wanted to make it a slimmer a-line silhouette, but I wouldn't try turning it into a mermaid. The alterations would cost so much that you'd be far better off selling it for what you can and buying an actual mermaid dress if this is the route you decide to go down. (Plus you might find that the bodice looks odd on top of a mermaid skirt since it was designed for an a-line)

I think you would look stunning in this dress (as it is, or with no train) on your wedding day, but if you're not happy with it, there's no harm in trying on a few more styles before deciding whether or not to stick with the dress you have.