How do I help Infinity understand I'm OK with how he feels and that he doesn't have to hide affection for Mustang from me out of fear I will not be comfortable with it? Infinity has a big fear that one day I'm going to become jealous or something and force him to pick between me and Mustang or that I will just leave. I have explained I'm not going to do that. That is not love in my opinion.
I don't run from hard stuff or feelings. I have been ok with seeing affection between them. It makes me happy to see someone else loves Infinity as much as I do even if they show their love in a different way from me. I am happy to see that Infinity lovest Mustang as much as I do even if it is in a different way from me. I have not felt jealous or envious of them and their time together when they get it. Why would I? These are two people I care very deeply for. I accept them as they are.
I don't want them to feel they can't be open around me or to constantly worry I'm going to switch gears and that they have to protect me. I'm mature enough that I know if something happens and I start feeling upset that my feelings are mine. I'm mature enough that I know I have to communicate with them and us talk and work things out as a whole. I'm not going to arbitrarily throw restrictions or anything like that at them just so I feel ok. That's not how this works.
Perhaps you could start with that last paragraph by telling them what you wrote in a journal- your thoughts and feelings. Maybe even read that paragraph out loud?
You could start with telling them
1) You love them both
2) you trust them both
4) you want to build on that trust with a specific way to make sure they feel safe and can trust you.
You could also make a SMART list of ways they can feel more secure and reassured other than just time.
Smart goals can be applied to anything, really useful I found in relationships, they are explained like this:
Specific
Measureable
Attainable
Realistic
Timed.
Gabrielle described trust to me like a marble jar, that we deposit in more marbles every time we are open and vulnerable and trusting towards a person. Each person has your own marble jar for them and different ways to earn or lose trust. As he described it we lose the marbles every time someone doesn't trust us, or hurts us, breaks a promise..etc. (I know this seems very basic, but going back to the basics is super important with Poly as all mono things are thrown under the bus.)
Do you feel by them not taking you at their word they don't trust you because of this quote:
"I don't want them to feel they can't be open around me or to constantly worry I'm going to switch gears and that they have to protect me."
Could you ask them if there's any specific reason for their lack of trust?
Or specific actions on your end that are perhaps subconscious and you don't realise?
If none of those things are happing, might be a good idea to see if you can ask for more trust from both of them, and also work on observing and not judging this new phase for them, it's going to take time to settle, so any time you can be calm and cool about it (Not worrying about their worrying) will add to those marbles in the jar.
If you worry they worry all the time, they'll see that in your behaviour it'll almost be like a self-fulfilling prophecy.
Since you specifically asked for advice here's what I got. Hope its some help.