Hi, Sunhorse99,
Look, I am no expert here but I'll do my best to give you my best advice, without sounding like a know-it-all just because I am older than you...
Actually I understand you and I think it is admirable that you want to have a nice relationship with your mother, letting her know about your life. Family is important and, specially when we are teenagers, our mothers are usually are our 'safe place'.
If you don't tell her anything and just keep hanging out with the two boys she might judge you badly and also dislike Phaelen imagining that he is sneaking into a relationship that was great before, right?
So I think being honest is the best thing to do. Tell her the truth... You don't need to use labels, concepts, definitions... Just explain her the situation. Tell her you still like your boyfriend but now you also have feelings for Phaelen and the three of you talked and decided to figure out your feelings in a very honest way, without hiding anything, without cheating, without hurting anyone.
She might not remember it right now, but she was a teenager! And one day she had to figure out her feelings, so I hope she understand that you are trying to do the same, in a very nice way, without playing other people's feelings.
Mothers will always love their kids and want them to be happy. Just show her that it is what makes you happy now and tell her that having her by your side is important to you.
Well... I think I said/wrote too much, sorry!
Wish everything goes well...