I'm in the tutorial looking for more resources, and I can't access part of the island due to a fountain I already visited. It's blocking the part of the island I need to get to. Is there a way to reset things or I'm completely stuck?

The second tutorial island supposes fighting monsters in order to access the whole island. The fountain itself doesn't block any parts of the island, but you can't pass further until you defeat the forest monster guarding a passage near the fountain.

I'm still stuck, I've defeated all the monsters I can reach. There's something behind the portal, but that's out of reach too.

Here's a screenshot. I've done everything I can do.

Go to the right then down if you can

You need to collect the cuttlefish in the water to proceed. It's not blocked by anything. Move your pet to the portal tile an click up once more to collect it.