Can't tell much from the screenshot. But use your healing grenade and sleep dart mostly in emergency situations to defend yourself to stay alive longer and not die so often.
Also combo your nano boost with dps ultimates like soldier or genji.
Like others said already try to get better aim. I don't know your settings, maybe lower sensitivity to be more consistent in hitting shots.
And most important tip: just play more, you'll get better over time by yourself
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Happy New year