God will never leave us because He has placed His Holy Spirit within us, to be present with us forever. In the garden of Eden, God walked with Adam, but now He has taken up residence within us, and there will never again be even a second we are without Him. There might be some days where you feel the sweet presence of His Spirit, and other days that you don't, but your feelings don't determine your reality; if God has promised never to leave, then you can always rely that He is with you, no matter what you're feeling in the moment.
God will never forsake us, meaning He will never turn His heart away from us. It could be possible for a companion to be physically present with us, and yet far away from us in their heart. God's promise is that not only will He always be present, but His heart will always be towards us. If you're going through a difficult time, you might be tempted to think that God's heart has turned away from you for a season, but He promises that no matter our circumstances, His heart is towards us and for us, that He is leading us through this dark tunnel to bring us to a better place.
Our feelings are like a child in the grocery store: one day they're pleasant and cooperative, reminding us of God's nearness, and another day they're kicking and screaming on the floor, telling us we've never known Him and questioning His love and existence. It's important to remember not to give in to the temper tantrum, but to stick to your errands, even if it means dragging a crying, screaming kid through the store. Temper tantrums end, and so do these dark moments of the soul. You will once again emerge into the light to be comforted by His Spirit, which has been with you and for you all along.