I take it you're not entirely happy being in a polyamorous situation per se, however because you want to be with DC, and you want him to be happy, you're tolerating his (married) relationship with DI. Correct me if I am misreading anything here.
That's a tough situation to be in. Do you and DI have a platonic relationship at all? Are you on friendly terms with each other, or is there quite a bit of tension/conflict there?
There's many poly situations where the persons involved didn't "set out to become poly;" they just found themselves in a poly situation. I've kind of gone down that road myself. But I get the impression you've struggled with it (even though you do consider it to be worth it).
FWIW, you should be able to read about many poly situations here on the various threads/boards, and you can post anytime you have a thought, question, or concern.
I just hope that you, too, can turn out to be happy (in spite of the obstacles).
Love means never having to say, "Put down that meat cleaver!"