Both of them made many mistakes and while Ennis' were more obvious, Jack had his mistakes too. He was afraid to be rejected, cos he loved Ennis so much, that he lost too many chances during years. 
Interesting discussion going on. I am not sure if Jack is afraid of rejection. He gets rejected time and again with other men, with sales pitch that don't work, with dreams that never go anywhere.
I think Jack's fear is deeper than rejection because he knows that Ennis already accepted him as he is.
Jack's fear, in my opinion is loss. He is afraid of losing Ennis altogether. He can't never quit Ennis because he never wants to lose Ennis. That's the love that they shared. Ennis and Jack loved each other so much, that either one of them losing the other is unimaginable and deeply painful. It's why they were miserable apart, ecstatic together. It is why Ennis suffered so after Jack died, and so comforted after the two shirts. They were finally together again.
Jack knew and understood their two souls as one; Ennis felt it and kept it at a distance, until too late. Then all he had were memories and dreams. In a way, it speaks to the depth and greatness of their love, that even death could not separate the bond between them. Even through death, their love binds Ennis to the memories and dreams that were Jack. In a sense, Jack's fear was unfounded because, if he had enough faith in the love, he would have known that it would not fail, that he would never lose Ennis.
Both of them had too much fears separately, and not enough faith in the love that they both shared in abundance.