If you have an app open and literally playing and then hit the home button, it will always pick up from you left off when you relaunch the app. Crave, Netflix etc behave that way. Always back out of the event your doing by using the back button. Bring the app back to its own (home) screen and then hit the home button on the remote to change apps or your selection of the next activity. This should fix your issue.
The onboard memory is at capacity. There are a couple of reasons for this, my first guess is a slow internet connection and the FireStick is buffering a lot of content.
Download an app called “Background”, it will let you close apps running in background and speed things back up. Lots of info online if you search “speed up FireStick”
If you look at the remote you will see the home button (looks like a little house) and the button directly to the left is the back button, just use this to back out of a currently watched show to reveal other shows on Netflix and Amazon Prime. This also works the same on Cyberflix and Bee TV