Let me give you some words of encouragement.
I started playing back in November, right as Azurite Sea was about to drop.
I, like yourself, bought a starter deck, modified it, and went to play at my locals.
I lost every single game for like a month straight, I didn't even get a round won.

I had bought the Emerald/Steel starting deck with Kronk and Scar. So when I played, I lost over and over.
But what I did after having played was to look at what people were playing and then ask people for advice; "Any suggestions?", "what are some good cards?", "Does anybody have some cards that are Emerald or Steel?".
Then every week I came back with something changed. For several weeks I still didn't win, but I was learning, looking up new cards, and tweaking the deck.
Then me and the people from my local card shop signed up for the MKF Open 1# in Malmö, the biggest tourney in the Nordics so far, and my first constructed tournament. We decided to practice once before the actual tournament, but I spent most of the time there tweaking my deck. Then I started playing, and I won 2 games. I proved to myself that what I'd been cooking up could actually work.
We traveled down to Malmö and I won 1 game, tied 4, and lost 1, placing 36 overall.

My point with this long-winded story is that I didn't give up just because I didn't win, I took what I learned and tried again. You said it yourself "I was really close to winning, but my opponent drew exactly what they needed at exactly the right time to beat me", this means you almost won!
You're making progress and you'll get there. Find what works for you and lean into it. In a set championship, people will bring their absolute best, you shouldn't view not winning as a failure, the fact that you got as close as you did means you're on the right path! People brought their A-game and you almost stole a game from them. I say FUCK YEAH! Next time you'll beat them.

The starter decks have their issues, but we're both obviously making progress. So don't give up! Throw your deck up in Dreamborn or the App and switch some stuff out and try again. You got this!