I'm sure it hurts, and I'm sure it matters to you a lot. To immigration - not so much. Marriages fall apart sometimes, unfortunately. And cheating / infidelity is pretty common, sadly.
Unless you have clear evidence she never loved you and only used you to get an immigration status - there's not much you can do. Even in this case, if you have evidence of her marrying for GC, the logical question would be: why did you stay married knowing it was the case? Did you involve in immigration fraud? This is a possible thread of thought in somebody's head at USCIS if you were to accuse her of this.
Now, to affidavit of support. Yes, you signed those papers and you're still liable. From what I understand, if she has an income of $24,650 or more a year, you don't have to write any checks. I'd think you need to be in contact with her to write any check if required. How else would you be fulfilling the obligation? Unless she comes to you asking for it, I won't sweat too much.
~ Not a legal advice ~