x-post r/relationships: I [21F] am in a relationship with a polyamorous man [29M] but I'm monogamous and I'm kinda panicked that I don't know what I'm doing?

So I have been seeing this guy, let's call him C, for maybe 2 months now? We've been having a good time, but the relationship is casual, mostly we hook up and then go do stuff like see a movie or something. I'm comfortable with the current arrangement of the relationship; he does have another partner (who he's been with for 6 years) and I've met her, she's quite lovely. My issue has more to do with my monogamy than his polyamory...

So we met on OkCupid, and I've left my profile active even after hooking up with C. I still check in every once in a while, and I have been responding to some messages. But one of them has asked me out, and I agreed before really thinking about it, and now I'm worried. I am very much monogamous; I do not have the emotional energy to have more than one partner at a time. So if I start dating other guys, I'll probably have to leave C, with kinda sucks. But I also have to consider that telling the guys I go out with that I am currently in an open relationship might be a problem.

Then there's the issue of my mother. My mother is a Christian African woman who is liberal enough to support gay rights and abortion but believes "atheist" is a bad word and is totally baffled by the millennial trend of "casual dating". I literally cannot tell her that C is polyamorous, because she can just barely deal with the fact that he is older than me and I don't have any real commitment to him. I live with my mom, so my going and comings out of the house are noted and inquired after (basically, I can't "secretly" date other people when she thinks I'm still with C).

And then there's my issue. As I said, I am strictly monogamous. However, the relationship I have with C right now is good enough for me - casual, no commitment, just kinda hanging out and sleeping together every so often. I'm in a period of flux in my life, I've got a lot of stuff going on in the upcoming semester (5 classes, part-time job) plus I'm going to transfer to another school (I haven't decided which yet T^T), so having a casual relationship is great for me.

I didn't start dating/having sex until April of last year (2016!), so it's kinda been a crash course in relationships so far. But I guess my point is, part of me is still searching for a "traditional" relationship (exclusive, monogamous) while a larger part of me knows that I probably can't handle that type of relationship right now and enjoys the arrangement with C.

So what do I do? Should I continue to search for a monogamous relationship? Should I accept that I am in a relationship with C, and that if I'm truly monogamous I wouldn't be seeking a different partner? Is it unfair of me to continue to message other guys, knowing that I probably won't end up in a serious relationship with them like they're searching for? Should I tell all of this to my therapist?? Any advice is much appreciated!

TL;DR: I'm a monogamist in relationship with a polygamist and I am still interacting with other guys, but it is confusing me about the nature of my relationship and about the fairness of me seeking out a different partner. pls halp.