Welcome to the forums and thank you for being here and sharing what's going on. I can see by your post that it probably wasn't an easy task. I feel so sad hearing what your roommate has been through, and is going through, but ultimately this post is about you.
I believe that what you're feeling isn't selfish - but completely normal given the circumstances. Both of your lives have been impacted by this car accident; both of you have had to adapt to a serious event, and both of you have had to readjust your own expectations.
In addition to all of that, you're living together - which means you're not only forced to cope with this, but somehow forced to cope somewhat together - even though both of you are coping and adapting separately. It sounds like your roomie is having trouble coping though, so I love the idea that both yourself and his family have suggested therapy. You did say in your post 'therapy again' - has he been in therapy before? What do you think is stopping him from going?
It sounds like the most difficult thing at the moment is him being a complainer, and I think that's where it's important to have some time for you. It's not easy being a soundboard, especially if he's aching for support rather than just someone to vent to. Some of the things that may help are boundaries, or even making time just for yourself so that you can recharge.
One resource that comes to mind too is Synapse- which is an organisation about traumatic brain injuries. They have super informative fact sheets and ideas (especially the one about emotional liability, supporting a loved one, self-care or challenging behaviours) - https://synapse.org.au/information-services.aspx
I hope that this helps