I don’t think I can marry my fiance anymore, how do i tell him?

Dear Ella,

We’ve been engaged for a little over a year, but I feel different about him
now. I’m not even sure if I really loved him romantically in the first place. I just wanted it work so bad that I fooled myself into thinking I could do it. He’s very dear to me, don’t get me wrong, and the last I want to do is hurt him, but…
Okay, I’ve been going through a hard time in the past year and a half. I have clinical depression and I’ve been trying to find the right medication and dosage. I’m afraid I may have asked him to marry me under the delusion of infatuation. We were passionate at first, but now I can’t even imagine having sex with him. And it makes him cry a lot. I don’t know what to do. I hate to see him like this, and I can’t help feeling like he needs someone else–someone who will love him fully and do him right.

But he’s so insecure that he thinks he’s nothing without me. As for me, I’m 20 years old and I’m not even really sure who I am anymore. I’m not ready for a serious relationship, much less a marriage, and it’s been taking its toll on my GPA. We have all these plans and I hate to break them, but… I don’t feel like I can go through with this and live my life in peace. I don’t want to marry him and doom him to a life of unhappiness. I just wish I could make him see.
How do I tell him this?