Hi Dragonfly,
I'm sorry to hear that you and your primary are going through this. It's rough, and I definitely understand it. Ultimately, your primary is right - only he can work through his envy. It might realistically be that things are rough on your primary until he finds a secondary for himself. It's difficult to let go of envy when your primary probably has to deal with constant reminders (your phone buzzing, you talking about your secondary, you getting ready for dates, you going on dates, etc.).
In terms of your guilt, this is for you to work on. Try to let go of that yourself, rather than basing it on your primary's emotions. It's important that you own your own emotions too

It's compassionate to feel bad for your primary when you go on dates, but practice separating your feelings from his once you've experienced the empathy if you can.
One thing I've experienced which may resonate with the two of you is this:
the more I tell myself that I "should" be over my jealousy or envy, the worse I feel. The more my partner tells me that she wishes she could help me and that she's tried everything, the worse I feel. At least in my case, I feel like a failure when I can't get past jealousy. This leads to feelings of shame and inadequacy, which leads to further insecurity and low self-esteem, which leads to even more envy and jealousy.
The most comforting thing in the world to hear, in my opinion, is this:
"I understand what you are going through. Thank you for being open enough to share this with me. Allow yourself to feel what you feel. Things will click in their own time." I'd honestly love to hear this from my partner. I wonder if that might help you two.
Also, I believe it was GalaGirl who once told me that emotions are like the weather - rain is rain, wind is wind. Emotions just *are*. You wait until they pass. In the meantime, I'd recommend that your primary does all that he can do to bring joy into his own life. The better we feel about our own lives, the less envy can take hold.
I understand that your primary wants to meet someone and that he's struggling with not finding a suitable candidate. While he should of course continue to throw himself out there (and, as Cosmo so rightly said, remember that getting rid of anti-poly candidates is a GOOD thing), I don't think that alone will solve the problem. There's very likely going to be times when one of you is dating and the other isn't. Learning how to cope with that now is better than sticking a date-shaped bandaid over the problem.
It might be useful to research and practice some techniques for letting go of negative emotions in the moment. I say this because envy in particular can really eat a person up inside, and it's incredibly distressing to be stuck in a negative mind loop. I'll leave you with a handful of ideas to show to your primary:
- Put pen to paper. It actually works, at least in the short-term. Write feelings down as short, blameless statements ("I am sad that..." "I am unsatisfied with...", then toss 'em out. This exercise can help with simply accepting what we are feeling and letting it go for a while.
- Distraction. Do something that you can really, seriously sink your teeth into. Do this whenever feelings of envy are triggered.
- Being thankful. I'm not going to suggest anything as saccharine as "list 10 blessings in your life". I'm talking about getting real. When I find myself in the middle of a self-centered pity party, I think to myself, "There are people in the world without a house / without legs / without eyesight". When I get mopey about only having one partner while GF has three+, I think about the millions of entirely alone people in the world. I basically give myself a sharp slap in the face and tell myself to get over my damn first-world problems.
- Treat your mind as a hotel and your thoughts as guests. If envy comes knocking, do you want to entertain it for a while, or do you want to send it packing? There's no right or wrong - it's your hotel.
The following websites might also be useful to you both:
http://www.beyond-karma.com/how-to/how-to-stop-compulsive-thoughts-letting-go-in-four-steps/
https://inspiredeverymoment.wordpress.com/2011/11/23/how-to-stop-negative-thinking/