Texting boyfriend as another girl to see if he is cheating. Am I really wrong for this.. or?

I've been on and off with my boyfriend for the past 4 years. He moved to another state after a year & a half of us dating. We broke up for over a year & got back together on December. I went to visit him on the last week of April but before that I downloaded an app for private texting & began to text him saying I was a girl he had met the past summer. The reason I did this was because I was unsure if he would talk to other girls behind my back. (Apparently he doesn't mind doing so but would get bothered if I did) So when I was with him I told him to let me see his phone & showed him the same messages I had sent him (as the other girl) & tried to have him explain why he was texting her... As the other girl, I would text him by being flirtatious & he would respond agreeing to meet up "whenever she came to town" which was "in a few weeks." At first he told me that she was just asking if they could continue texting (he was obviously lying). We kinda fixed that "problem" & he said he wouldn't text her anymore or any girl that was flirting with him in any way or any girl from the time we were not dating. A few weeks have passed & I notice him a little different so I decided to text him as the other girl again. Of course he keeps replying & asking for her snapchat, instagram, Facebook & a pic. She is supposed to be in town this weekend & they're going to "hang out" Saturday night. I know the with this girl nothing is going to happen because it's obviously not her texting him. But it makes me wonder if he talks to anyone else or what would actually happen if this girl were to actually be there one day. Would he really hang out with them behind my back? Even after he promised he would have no type of contact with any other "friend" (as he calls it) of his. I know I'm doing wrong with this but I wanted to see if he would actually keep his promise or not (hurts that he's lying). Specially because he is definitely not ok with me doing the same. Someone please HELP me!
Texting boyfriend as another girl to see if he is cheating. Am I really wrong for this.. or?
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