These are broad tips for beginners coming from casual play throughs, so some of this you might already know, but here’s a few things that come to mind:
1) it’s ok to feel discouraged. VGC can be quite volatile, especially in a best of 1 format. Just don’t take losses too personally, they are an important part of the process.
2) don’t judge your current team too harshly based on if it wins or not. All teams have weaknesses and you should expect to lose often to those match ups. Instead, ask whether your intentions are working out. For example, in the last game did you lose because your opponent had a good match up or a lucky read? Or did everything go according to plan, but it just wasn’t as effective of a plan as you thought?
3) The fact that you are messing around so much with team composition before making it to master ball makes me think you need to work on your doubles battle skills more. The pace in VGC is very different from singles and doing otherwise common things like switching too much can cause you problems. It would probably be helpful to pick a team and stick with it for like 20-30 games. You will become painfully aware of the weaknesses, but I’ve seen some real dumb teams make it to master ball simply by good piloting. Working with your weaknesses rather than hiding from them by constantly changing up the team will make you a better battler and will also help you gain perspective on why you lost.
4) being able to counter every viable lead is probably not realistic. Like you said, there’s just too many. But that style of play is very reactionary and often puts you at an immediate disadvantage. With how fast paced VGC is, by the time you reposition your side to counter, your opponent has significantly chipped your team. It’s often better to just go for your own game plan, get some momentum, and force your opponent to react to you. In other words, prioritize your game plan or win condition, then figure out what counters or adjustments you need to make.
Here’s an example of what I mean by game plan. One of my favorite leads in series 1 is fake out, close combat hariyama and trick room, hyper voice farigaraf with throat spray, with eruption torkoal and sucker punch meowscarada in the back. The plan: get trick room up turn 1, and do damage until either mon faints, then send in tera fire torkoal and destroy everything with eruption. Grass cat in the back comes out and can either sucker punch or protect until trick room expires, then clean up the end game with flower tricks.
This plan is not perfect. Indeedee, armarouge, ceruledge, gholdengo, and many others can all throw a wrench in things, yet I almost always start out with trick room fake out. Maybe add knock off to hariyama or give farigaraf sash, but I learned so much about the philosophy of VGC by focusing on my game plan, rather than the counters.
5) like others have said, take breaks and have fun. There are plenty of ways to enjoy VGC that don’t hyper focus on winning.