Hi BrookeCAP. Welcome to our lovely forums.
My main advice is, don't start with the entrance and mainstreet. Especially if you're thinking of the scale like a Disney park.
I think that what's happening is that you're putting so much time and energy into the entrance and mainstreet, that you're burnt out before you get chance to do the fun stuff like building rides. And that's the thing. A mainstreet is a LOT of work if you want anything half way decent.
So here is how I'd try it.
Sketch out where you want the entrance and main street to be. Terrain paints are dead handy for this. Also making some basic building footprints and plopping them down really helps you visualise the scale and length of it as well as giving you an idea of how many buildings will be on your main street. Once you've done that, leave it and move onto one of your areas.
Start small. Look at images of themeparks, buildings, paintings, etc for inspiration. Start with a building or coaster or a ride. Add some terraforming and planting. As soon as it starts to look like an actual area, you'll be surprised how much inspiration it will give you and how quickly it will develop. Once the ideas start to flow, you'll be wanting to start other areas.
You can always dip in and out of working on your main street and entrance while working on the other areas of your park.
Letting your park grow organically and going with the flow as you get ideas will keep it fresh and fun and will help keep you motivated. As soon as you plan an entire park, you're putting pressure on yourself. It's a daunting prospect to fill a map and finish a park. It can also limit creativity as you can become a slave to your original plan even if you get new ideas.
So what I'm saying is let loose and enjoy yourself. Build for your own enjoyment, not what you think others will like. Don't pressure yourself to finish anything either. If a project stops making you want to go back to it, don't force it.
I hope this helps and I can't wait to see what you come up with.