Something I like to do after finishing a long book is to switch over to a book with fewer pages and an "easy read". For example, I'm still reading the Harry Potter Series in Spanish and just finished Book 6. Being that it took me months to read, I swithced over to a children's book (for ages 7–12). I'm halfway through this book after a few days. The book is about 10X shorter than one of those Harry Potter books, the print is larger, and I don't have to look up too many words. I actually read the first book around the first year (it's a 3-book series) I started reading in my target language, and it was a much tougher read back then. Thankfully after doing daily reading, more and more books are easier to read.
There's a reason why a lot of people aren't avid readers and may struggle learning their target language on a deeper level. In their minds, they link reading to boring homework they had to put up with in school. In reality, pleasure reading is what makes people speak with a better command of whatever language they're speaking… including their mother tongue.
If you're reading this, chances are you're an English speaker. You don't have a good command of the English language because you speak a lot and watch a lot of TV in English, it's because you've been to school and done assignment in English constantly. Every class you've had to attend, you needed to read and write in English. You had to listen to class lectures in English, and you even had to do boring homework in English. Imagine how extensive your English vocabulary would be if you took pleasure in reading. This is why you can go really far in your target language because the reading, writing, and listening activities can be done ONLY in things you enjoy. There is no such thing as a "boring reading assignment". If it bores you, you don't read it. There's no deadline so you could take as long as you want reading one book.
Getting back on topic, after doing some intensive reading of a book that may have a lot of unknown words and has hundreds or even thousands of pages, the next book you may want to read is a book that's under 200 pages and is an easy read with large print.
Finishing books gives you a sense of accomplishment. If it took you 3 months to finish a book due to difficulty and page count, it feels good to finish a book in a few days before reading another book that will take you another 3 months to complete.
As the years go by, and your reading speed increases, you'll be able to finish larger books in a few days or a few weeks instead of months. When your vocabulary increases, you'll have to look up fewer and fewer words which saves you a lot of time looking words up. You can switch from your eReader to physical books because you know about 99% of the words you read because your vocabulary has grown over the years.
Don't feel like you're dumb because you want to read children's books after reading a heavy novel for adults. Keep in mind that children's books still have words you may not understand as an adult-aged person. Keep in mind that you're still engaging in the activity of reading. Just like going for a light swim is still a form of exercise, just like lifting heavy weights.