KenoLeon:
Learn the lesson and move on.
Don’t be too hard on yourself, even if you had a contract (which I recommend for next time), the legal and time costs might make it not worth pursuing the claim.
It is my opinion that as designers/creators we tend to believe the product is everything, but in business I’ve come to realize that finding the right people to work with is more important than the product you are trying to sell.
Another thing that crosses my mind: He just validated your design by stealing it, why don’t you try selling it by yourself ? I would use something like represent no upfront costs and they specialize in t-shirts , It might go nowhere, but if you share the story of how it got stolen you might even get more interest.
Well said “KenoLeon”