When it comes to collaborative writings efforts, they’re really convenient for consistent progress, as both authors can add to them over time, and you have someone for idea bouncing and sanity checking. That said, nothing is as frustrating as when your partner hits a creative block in the middle of the 2nd Act. You don’t want to forge ahead alone, and you don’t want to shelf it for a solo project, because who knows when you’ll be able to unshelf it?
Personally, I see writing as a solo game. Sure, you might be having fun with your friend, but I could never work like that. Why don't you want to forge ahead alone? That might be the best thing for you as a writer.
I feel ya. It's been a while back, now, but I used to have a writing buddy I clicked really well with, and we were both always really excited to share new pages ... until she wasn't anymore. We put it on hiatus. Then we eventually drifted apart entirely. It's a bummer. It's difficult to find someone you can work with on a creative project anyway, but two people sustaining interest and drive -- and having the time! -- is just extra layers of complication. I'd be willing to try again, but for now writing is solo for me. I did end up scavenging some inspiration/concepts from our joint work together, though -- all my IP, of course, if it was taken more directly.
Collaboration is all about energy - when it's going well, you can feed off each other. But when it's going poorly? Yikes. I don't really have a solution, but wanted you to know you're not alone!
I'm waaaaayy too much of a control freak to collaborate with another writer. I know plenty of people who have done it successfully, but with my personality it's a no-go.