I know it's confusing and hard to accept, but first, make sure that you were tested for herpes in 2012. You may not have been - it's not part of routine STD testing now, and sure wasn't in 2012.
Also, make sure that the test you got was type specific, and differentiates between type 1 and type 2, that you have separate results for each, and make sure it's an IgG test.
If it is all of those things, and you were tested correctly, and tested negative in 2012, and the retest confirms your infection, then you were indeed infected sometime between 2012 and now. However, this doesn't mean your boyfriend cheated. It's quite possible that he had it and didn't know - 90% of those with herpes don't know they have it because their symptoms are so mild they don't recognize them as herpes, or they get no symptoms. He could have had it prior to your relationship, and didn't know.
If you are indeed positive, and this is the only indication that your boyfriend may have cheated, wait to see what he says. It's not enough info to state for sure that he did. I wonder if you already suspect something, or you probably wouldn't be testing for herpes (and other STDs?) years into a relationship when you aren't symptomatic, unless you are pregnant or something.
You should both read the Herpes Handbook at https://westoverheights.com/wp-content/uploads/2014/08/Updated-Herpes-Book.pdf. It's free and written by one of the world's leading herpes experts.
Good luck, and let us know what your retest says.