Depressed..should i stay or should i go??

tina my boyfriends x-girlfriend kissed him rite infront of me he kissed her back im so confused because he said somthing like "if i didnt kiss her back i would never know if i still had feelings for her and now that did, i know that you're the one for me sammy" should i belive him..after all he's never lied to me. the other problem is..i took things into my own hands and kissed another guy cause i was pissed..and he never saw me do that..so should i tell him or?