Fire Emblem heroes is a game made to be played everyday for like 20-30 minutes. I've been playing for 8 months now, and I'm still trying to finish the entire chain challenges in Lunatic difficulty (nearly done with btw).

What I'm trying to say is, do not expect beating people who played for a while in only 2 weeks. Stay focused on what you what you can do for now, and do not rush you ascension in the arena or AR. FEH is really fair if you take the time to understand everything. Also, I never spent a single dollar, and I'm tier 20 on the arena and 16 in AR. Just be patient, and appreciate what you can do with what the game gives you. Imo, the most important thing you need to understand is how to spend smartly your orbs. It's your only way to gain better units, so take your time to understand how it works. It's not only luck, there are a lot of strategies behind.

Good luck, and enjoy! :)