The best thing I can tell you to do is play a better card game like Yu-Gi-Oh or MtG. While I don't think Yu-Gi-Oh is a much better card game, it did teach me fundamentals. In my many years of playing it, one lesson I was always taught was to NEVER overextend. Each time I played something I didn't need to play, I would either lose outright, lose board state or risk something I didn't otherwise needed to. A example of this was when I had game point, I summoned a monster and when I did, they activated their Trap card Torrential Tribute, which destroys all monsters on the field, costing me the match and I would do things like this often. The fact I played these games, it made me a better Pokemon player. The same thing is true for fighting games. The things I learned from Mortal Kombat, Tekken and Street Fighter made be a better player in other fighters.
I don't play anything I don't need to now and I don't ever overextend. With cards like N, Red Card, Delinquent and the many other hand disruptions in the game, I wouldn't ever play a Tapu Lele for a Supporter I didn't intend on playing that turn, though this does depend of the Supporter I plan on playing but even then, doing that gives the opponent too much information, something you don't want to do.
You also want to assume somethings. I always assume Dark decks play Delinquent, as well as Quad decks and other disruption decks and don't let my had drop below four cards if I can avoid it. Believe it or not, Garbodor is a disruption deck and the do run it, sometimes two copies of it so they can force you to discard Item cards. Sometimes you do want to grab a Supporter to deck thin so you have a better chance of drawing something else. The goal here is to just play a deck and continue to play it until you know it. You'll figure out and learn things as you go and sometimes it can take months or years to learn the deck. Dark decks are as good as they are because of how many people played them and the tech learned. It's not going to be easy but you will learn. Don't stop playing a deck because things don't go your way, just use it as a chance to learn.
