Perhaps a male perspective would help...
When young men encounter stress they often react by going into themselves and reacting with, while not aggression, a very unfriendly manner. This is not a wonderful or even healthy way to handle issues, but it is very male and very typical. The distancing that occurs is designed to shut out all external influences and to allow time to make some choices. Until those choices are made the man usually acts as you describe here, with lots of push back and distancing.
The stage actually has a name. It is called moratorium and is a time when critical choices occur for a male
What can you do as his girlfriend?
Clearly he needs some time and distance to make these hard choices. This is especially hard for women to accept as when these types of issues occur in a woman's life she often will run to bond and talk with others, the exact opposite of what is occurring here.
It may go against your very nature, but accepting your boyfriend's need to take some time to choose his path is exactly what he needs. Really, although it may seem that your past and your "wrongs" are the issue with him that is just his way of creating space between you as he knows you are unlikely to want to talk about these things in light of what you both agreed upon earlier.
Best practice: Tell him that you know he is going through a rough time and that you will be there for him. Tell him that you know he is under a lot of pressure and that your relationship should not be a part of that pressure. Tell him you are going to give him some time to think about what he needs to do; but that you will be there are care for him. Then allow him the time to do so.
This stage is a difficult one to go through with a man, but it is common enough and allowing him the time he needs to make choices greatly improves the chances that he will come out of this even closer to you than before. But, as most men have no idea how to ask for this space (we are kind of primitive!)...the push back you are getting is his way of getting it. Ignore it as personal. See this as a period of maturing for him and as long as you tell him you will be there for him, you have done all that you need to.
These things usually work through, but he does need the time to process these choices and know...this is not about you. He will come around but not until he has the time to work this out. Steven