i imagine it must be very hard for your mom.
i had to google to see what those shirts looked like (i'm obviously not familiar with them)
they meant a lot to you (in part b/c they meant a lot to your dad--which was all your fault to begin with because you started it! lol) . i think you should go out and buy yourself one and just tell your dad: "i really want to get you another one of these shirts, i miss seeing you wear them, and i know you loved them. but since you won't be wearing *this* one, i'm going to wear it for you. so i have to make sure it's my size."
tell him! go get a shirt. you want to feel close to him sometimes then put the shirt on!
i'm sure he would be rather amused if you did that.
and it might even help be a bit of a comfort to you. (it's okay that it's not one of his old shirts--it would have been nice, but you would have gotten him another one eventually anyway. make that one a shirt you can wear.)
i'm glad you spent some time talking with your mom. and i understand how emotional it can be.
my mom passed away coming up on six years. i'm not adapting/adjusting like most people are able to do--but that's okay. i probably would have done well with grief counseling but i wasn't able to get help. (it's complicated.) but if you are able to get some--even in a group setting--it might be extremely beneficial.
when my dad passed--years and years ago--his church offered group counseling to the family members and although *i* didn't belong to the church i went and it was very helpful for me.
take care.