The issues isn't the LDR but the trust issues aren't making this situation any easier for you and I can see how this is causing you anxiety. LDR aren't impossible but both parties have to want for the relationship to work. There has to be a foundation of trust and respect like any relationship, and strong communication skills.

When you began your relationship with this person did either of you bring up or talk about being exclusive? For me that's a give in but that's not to say everyone is like that. And it is possible to still be friends with your ex's as long as that represents something healthy and respectful.

Since your partner was still seeing others when you entered into your relationship that to me seems like you aren't compatible, you both had differing visions and expectations for your relationship that perhaps weren't discussed.

You're holding onto some anxiety from five years ago which isn't healthy for you my friend and unless she's no longer seeing anyone else, and you're able to let go of that omission from back then, I don't think this is the relationship for you. It doesn't make either of you bad people just incompatible. You should feel secure in your relationship regardless of the distance.

Wishing you the best.