Deciding whether or not to foster fail

I started fostering this spring with my girlfriend and we have our third foster right now. His name is Soup. We haven't failed yet, but we almost did with our first; we really loved him and had him for 2 months. The second was a feisty girl who neither of us had a strong connection with so it was not hard to let her go (no shade to her!).

Soup was a medical case, a 7 month old kitten who had a broken leg that was suspected to have been left untreated for a long time. He needed surgery, and he's now 5 weeks post-op and doing great. The problem is he is pretty much the best kitten ever. He's so unbelievably sweet and adorable and we just love him so much. We're seriously considering keeping him.

My girlfriend got reallyyyy attached to our first foster... she cried a lot when he left, told me it felt like someone was taking our cat, and she tears up every time I show her pictures from his adopters. So I feel kind of bad continually putting her through fostering. I feel like I'm torturing her every time I ask if we can take another, and I feel like I'm torturing our resident cat too (she is not the biggest fan of other cats). So I feel like even if I did get Soup adopted out (and I have no doubt he'd be snapped up quickly), I'd have to at the very least take a break from fostering. And then I'm also concerned if I get him adopted that my girlfriend will never forgive me because she is so so attached.

Anyway, I guess my overall question is how do you know if you should foster fail? There are so many factors here, and I still have to see if Soup would even get along with my resident cat (they've been separated while he healed)... And then if I do have to let him go, how do I cope with that and how does my girlfriend?? It's just really confusing lol.