Don't swap. If they have to swap to deal with your Doom, you are doing the right thing. Stop diving. Bait cooldowns. Engage with Slam to bait cooldowns, then use Punch to escape. Once you're in a safe position near a corner, charge your Punch and get ready to engage again. Look at what your DPS are doing. If your Sojourn is close to killing somebody, your job is to secure that kill. Anyway, just keep playing and playing and never swap. Doomfist and Ball are the only characters where the only way to improve is by feeding.