ever since i left Islam I've been depressed.

i dont know what do anymore
Hey brother, we live in truly confusing times, you must understand that our Iman or faith is not linear, rather it fluctuates from time to time depending on our closeness to Allah , our situation and experiences. We all have times where we go through inner struggles and question our selves and our faith, this is true of all humans and how Allah has created us.
Nevertheless we are also created with a default love for Allah/God, its why when we are in trouble we call out to him, even the atheist, its buried deep within our heart's and it manifests clearly the existence of Allah. What atheist try to do is block this, knowing full well the truth, which eventually leads to guilt. This guilt leads to grief and depression, its why many atheist suffer in silence and live miserable lives.
However all is not lost, Allah is the most-merciful, most-forgiving, and if you repent with sincerity all your bad deeds will be wiped out. Start small and work your way up, but remember all sins can be forgiven, expect Shirk ( denying the existence of Allah or association partners with Allah).
Islam is beautiful and easy, so seek knowledge whilst you have the time.