As someone who has been collecting cards for nearly two decades, let me share some of what I have learned along the way:
I almost always buy two of every deck. This is not always an option (see the new Gold Monarchs), but I feel better about opening a deck if I know I have a backup for later. There are some decks that I want to own, but do not want to necessarily use and in cases like this I will only buy one deck. As you start to collect more and more you will find you will begin to be more selective about the decks you own. Cost is one reason. Storage is another.
I used to display my decks on some picture ledges I got from Ikea. You can find them for sale at other stores, but Ikea was the first place I noticed them so I just got more and more shelves as my collection grew.
Here's a picture of what it used to look like from my Instagram.
Having just moved to a new city, we are renting and need permission to mount any kind of shelving for display. At the moment I keep my decks in boxes. You can find great fitting boxes at comic book & gaming stores for a buck or two. These are plain white cardboard boxes built for cards like Magic the Gathering and trading cards. They come in all kinds of sizes, from single rows to quad-rows, but I have found two rows (about shoebox size) is the best route to go. Too big and it will be cumbersome and heavy to move and store. Too small and it defeats the purpose of storage. I got the idea by watching this video:
Finally, be happy. Don't beat yourself up about the cards that got away. Collecting cards shouldn't be a stressful hobby. There are always going to be cards you feel like you can't live without and then another design comes along that makes you happy again. Enjoy it.