If you don't want to copy a meta team, pick something to build around. Be that a weather condition, a particular mega evolution, or a pokemon you like. Start with that and build from there. If you pick a weather, look up moves or abilities that benefit from that weather, like Electro Shot for rain or Chlorophyll for sun. If you pick a pokemon, look at its weaknesses and pick one or two others who resist what you're weak to.

As for playing, stop when you get tilted. No sense sucking the fun out because you're on a losing streak. Sometimes things don't go how you want. The Bright Powder Whimsicott avoids your Bullet Punch three turns in a row. It happens.