Is this the first time you're donating to it? If you donate over the needed amount for your town "upgrade", the excess money doesn't go over to the next one. You also have to go in order. Bridge first, fountain second and lighthouse/windmill third.
The 3rd bridge is first, you need 200,000 bells for that. You will need to speak to Pelly the day after you have donated to vote for the location of the new bridge. A week later it will be built at the location you picked.
Next is the fountain. That one costs 400,000 bells. It'll also take a week for this one to be built. You can't choose where to build it either.
Finally, the lighthouse/windmill. You can only pick one and you also can't choose where it'll be placed. This one costs 1,000,000 bells. Like the others, it takes a week to be built. If you donate another 1,000,000 bells, then you can swap the one you have for the other one, but you can't have both at the same time.