you don't say exactly how he messed up, but if he is now trying hard, perhaps you do need to let go and try again yourself.
The miscarriage is a different matter though. That's quite a traumatic experience and you need time to heal , mentally as well as physically. He needs to understand that. I don't think this necessarily an unhealthy relationship - what you need to do, I think, is sit him down and explain exactly how hard the miscarriage hit you and how it will take time to mend. If he still keeps nagging you about it then, yes, it may be he's not the guy for you (or anyone else). But I'd say try to work it through first before you write the relationship off. You do need support at the moment and he may still be able to provide it if he realises how much it's needed.