You're not meant to win these. Everyone with a below endgame roster has a BGs cap, meaning we can't really go past that point without very lucky matchmaking. You've likely hit yours, just as I hit mine at D3. One thing I can recommend is not forfeighting. You can't expect to win fights if you quit them, and even the best players in the game can still slip up and pass out free wins at times. Play them out, learn from them, and good luck!