I'm 46. I came to Christ when I was 19. I, too, felt extreme highs from learning more and more- even praying the prayers of Ephesians for a few hours at a time for a season.
I, too, have been in a state of depression- for about six years or so. I have been let go from a number of jobs due to depression and ADHD combined. My life has been ruined no less than three times. Completely.
Regarding having nothing and no one? I always have been a member of a church. Always. I have not avoided loneliness, but I have minimized it by staying connected with brothers in Christ. I just showed up and kept showing up. I served and kept serving. No matter what.
What you are running into is something called dis-illusionment. Or something like it.
And yes, it says that God won't give us more than we can handle, but mother of pearl, that is not EVER how it has seemed in the midst of my trials- and there have been many. I too, have felt unworthy and hopeless. for extended periods of time. BUT... I looked at the cross of Christ- God's covenant with me in His very own Son.
Listen: God knew ahead of time that nothing LESS than sending His only Son to bear the wrath we deserved for all kinds of sins would deliver us. So, did He do less than that? No. He GAVE His Son. And, keeps on giving.
We are so bad off, God had to rescue us through His Son Jesus. He knew ahead of time that we could not handle this, so don't listen to the condemning thoughts in your head.
It's OK for you to be where you are. It won't last. Christ, Who is in you, will outlast your trials. That's not just my promise, but HIS promise to you. He would not have died for you and been resurrected if He were lying. He tells the truth. Don't let the deceitfulness of your unbelief win. Get a little angry, and fight for your belief the way Jesus Christ fought for you. He will never stop loving you. Period. Don't forget that. But remember this: His love does not rely on your feelings. Ever. It relies on His character- He is faithful- so faithful He cannot deny Himself.
You have isolated yourself, and the enemy has encouraged you to do so. Don't. Do the opposite. Go meet with a pastor and cry for a while. That's what they are there for.
I lost a brother in an accident when I was five. Blamed myself until I was 30. Fought through ridicule and isolation as a child and young adult, learned I was ADHD after many ADHD induce trials. The only thing that saved me wasn't a thing- it was a person. It was Jesus. He knows your trials and wants to not only walk through them with you, but wants to help you overcome.
Please message me. I will listen to you any time. ANY. TIME.
You are loved. Non-stop. Keep going.
Oh, and read Psalms 25 and 27, and all of Ephesians, Philippians and Colossians. (If you are that desperate, do what I say.)
I love you too, so hang in there. I don't want to find out that all my words were wasted! (and please remember that God has an amazing plan for you- I know it's true.)