I am sorry you struggle.
I don't know if it will help you, but I am going to flip your post upside down so you can see it that way.
Also, as a secondary dating a married man, how can I learn to respect myself?
I think you could stop holding back and actually
participate in this relationship if you care to be here. Stop shrinking yourself so much. It will either pan out or not pan out as any dating relationship would. But stop being this "shrunken, hanging back" version of yourself. That is not self honoring behavior nor self respectful behavior.
And stop "down talking" yourself like you are not a "real relationship" like his wife. If you feel or think like participating in a V like this makes you less real or disposable or whatever? Date 1:1 instead. Don't be in relationship models you don't actually like. That is respecting your own personal preferences/limitations.
How can I become ok with never being married myself if I stay with him?
If you want legal marriage, accept it's not gonna be with him. You can have commitment ceremonies with him, but not legal marriage. Bigamy in most places is a no-no.
So if you want to be married eventually? Start dating other people too. Ones who can offer you legal marriage. He cannot.
As a secondary, am I wrong in thinking he needs to put in the effort to keep me happy? Especially since I know he is uncomfortable with me seeing anyone else.
Where is he NOT doing enough effort? You do not actually say what you want more efforts in. How's he supposed to know you are not happy if you don't communicate? He cannot be a mind reader.
When you hang back and don't fully participate in this relationship are YOU putting in the effort to keep you happy?
I feel like he should always be making the effort to see me and make me feel important to him.
Why? You don't think relationships are a two-way street? Were both partners participate?
Is he the one devaluing you or are you the one devaluing you?
To me I'm hearing you avoid connecting with him because YOU think you are "less than." That's not him devaluing you.
Is there more to the story?

Does he devalue you somehow? I do not see from the original post examples of him doing that behavior.
Is this the wrong way of thinking? I’m not seeing anyone else. Only him so he is all I have. Are my expectations too high?
You seem to expect that your partner has to do all the work and "carry" you in a relationship while you hang back. Are those the expectations you have? If so, I don't think that's healthy or realistic.
This is what you list that you do:
- I don't give myself permission to love him with all my heart and just be happy.
- I hold back, because (I think) his true love belongs to someone else.
- I never ask to spend time with him. I wait until he wants to see me.
- If he doesn’t text me first, that usually means we just don’t text until he does.
So WHERE are you in this relationship? You don't sound very present in it from your behaviors. If it's basically a kind of "meh" relationship that you are kinda checking out on... why be there?
Finish checking out and end it. Make a decision.
I could be wrong in my impression. But to me it sounds like you want more time and connection but don't actually want to ask him. Then you feel sad and cry because you get lonely since you decided not to date anyone but him. Then tell yourself that he is busy with his real life and wife and too busy for you. Like "why bother asking him... he'd say no anyway" sort of thinking.
It also sounds kinda like you want him to be a mind reader and when he "fails" to do it "right" you get upset that way too.
Here's what you list that he does:
- He says he loves me.
- He wants to see me
- He says he is uncomfortable with me seeing anyone else.
The first two sound fine. He cares and he engages with you.
The third -- well... could talk it out with him. What does he need to BECOME comfortable? It may be you need to actually date other people. Like... he's willing to work on it, but you eventually have to actually date other people for him to actually get the practice of sharing your time and attention in real life rather than only in theory. Or is it that he wants an OPP?
I would suggest you start actually participating and engaging in this relationship if you want to be here rather than hang back and feel all sad he cannot be a mind reader. Actually ASK for what you want/need.
If you don't want to be here anymore because it feels meh? Then end it
properly and stop dragging it out. Your time is valuable. Could behave like it.
Date other people if you want to find a marriageable partner. He's not it. He can be in your poly network if you want to continue with him that way, but he's not ABLE to be the spouse person for you. Bigamy is not allowed.
Stop telling yourself you are "less than" like you just stink. Why be your own self bully? How is that self-respecting or self-honoring behavior?

It also doesn't sound like you have fun doing it.
You are allowed to take up the space you do in this world.
I hope things improve for you.
Galagirl