Hi Davidf01,
glad you're making some progress on your design and that you're getting some great tips from Jesse (jjurban55) as well as some other Autodeskers.
So I took a look at your design and a couple of observations:
my first one was that you are likely going to do some sort of polymer production process: rtv mold, injection mold, 3D print-maybe!?, but probably not thermo formed (too thick a part).
Considering the first two options mentioned above, I see a couple areas where your model will need fine tuning to get a proper mold made, and that is drafting your model to get a uniform finish on your part and to get the finished part out of your mold without marring your finished surfaces.
So I wanted to show you another way you can get the result I think you're after.
in the image below I'm showing a sketch that matches your design intent sans corner radii, as I added them as solid modeling features rather than sketch elements.
The one thing I like to share with beginner users is keep it simple.
Try and distill your sketches to the most simple, base geometric form whenever possible. In the case of your mug lid, I had forgone the relatively complex sketch in your model and just used a simple circle to create the first two features that are the base of the lid design.
Build your design as primitive geometry and use the power of the modeling features to add detail; rounds, draft, shell for part thickness, etc. Although your design is not too complex, keeping this sort of detail out of the sketch when you're working with more complex designs will serve you well if you have to troubleshoot a failed feature in more complex models. In time you'll learn how much detail you can add in a sketch to capture critical design intent and where you can rely on the 3D modeling features to get the intent you want, there is no magic formula here, but keeping it simple is a good place to start.
Feel free to play with the example I built and step back and forth through the timeline to see exactly the steps I took.
hope this helps.
p.s. the "hollow" look in your cross sections is from the ambient occlusion setting, turn this off and all should be good. Also to create a sketch on an existing surface the surface has to be planar, if the surface has any "out of flatness" you can't sketch on it.

hope this helps,
Jamie Gilchrist
Senior Principal Experience Designer