I'm not big on the kid comparison myself as I don't have sex with kids... it kind of weirds me out, but it is a close example all the same. There isn't really another I can think of.
Will you not be increasing trust because you want to talk to her about it. Can you market it too her that way. You are being honest about who you are, being patient with the process and with her in it, ready to do what it takes to make her feel loved, special, needed in your life... if you then show that you are serious, yet doing the things that you say you will do (show her you love her etc.) she will see that you are trustworthy.
The good thing about poly is that it can start a whole new romance with ones first partner. It shows up all the things that have been sliding or have slide in a mono relationship. Now is your chance to re-kindle with her. Make date nights, talk deeply about something, do the things you like to do together and involve her in your journey as well by keeping your willingness to communicate open.. It will be hard too for her, but there is nothing more you can do but be your authentic self.
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