I couldn't read this without replying. You sound so very, very sad and low.
First I want to send you virtual hugs and 
Am I always going to feel like I wasn't good enough...
You sound like a beautiful person in your posts; kind, generous in your love, fair minded, devoted, loyal, steadfast. But I wonder can you see what an amazing prize you are? You absolutely deserve a happy marriage. If you're guilty of anything it's not holding yourself in high esteem.
I'm terrified of the idea that it's not fixable. I don't want my marriage to be over
This might seem counter intuitive, but do you think you could face your fear and open up your imagination to what might it might be like not to be married anymore? Free association writing (where you write and write without pausing to consider spelling or grammar, just let it all spill out of you onto the page) might be a useful tool, or you could discuss it with a therapist. I'm not suggesting this as a first step to ending your marriage.
The terror you described comes across in your posts. It seems to me that it is standing in the way of your recovery. You don't dare push him away by expressing your feelings, you are wary of still bringing it up, of not letting it go, etc. But if you want to heal you might have to take the risk that this marriage won't survive an honest, and full expression of your pain.
he is as patient as I believe anyone capable of being with my apparent inability to let go of my hurt
It hurts because you haven't even begun to heal. Being patient is the very minimum you deserve. This isn't about any inability on your part. This isn't, and never has been about what you can or can't do, what you did or didn't do. please read the link cruusshed posted which might give you an inkling of the magnitude of care to which you are entitled, the magnitude of pain and grief you are entitled to express and the magnitude of guilt your DH has not yet begun to fathom. You are like a starving child, pathetically grateful for crumbs.
Please don't read what I've posted here as criticism of you, or of your efforts to salvage your marriage. You posted here wondering if it will always feel so bad, because you're three years on. But in a way you're not. Your pain and suffering got pushed to one side, so really you're only at the beginning. Even if all you had going on was raising a child, the first three years are so all-consuming that you'd only feel like you were coming up for air now anyway. You also put your needs to the side to support him through a bereavement, and that sounds to me like the decision of a compassionate, decent human being. Is there time for you now? Is this a good time to put your needs front and centre and sort this out? Because everything up to this point has been like putting plasters on a badly broken leg. If your child had a broken leg would you be satisfied with yourself for not being impatient when they can't walk fast, being patient when they cry with pain? Or would you be doing everything in your power to ease their suffering?
You seem so small in your posts, like you've curled yourself up tightly; you need to find your strength, claim your space, be BIG. You want a happy marriage then you absolutely need to start claiming your right to what happy wives have!
I don't understand what more he might do
This to me is the crux of the problem! Imagine for a moment that you cheated on your wife, punched a fist through her life, destroyed her trust, trampled on her self esteem, wiped your feet on her soul. Now imagine that person managed to draw on reserves of strength and compassion so deep that she not only stayed with you, but carried you through the worst experience of your life, and imagine coming out of the tunnel of your grief and looking at your child, having missed most of their life so far because you've been wrapped up in yourself. Imagine the gratitude you'd feel for another woman who raised your child while you couldn't? Now how would you begin to express your love and gratitude to this incredible person? What lengths would you go to prove yourself worthy?
You deserve so so so much more. I hope eventually you claim it!