I am so sorry that you had to let Jasper go. It sounded like the poor fellow was in great distress and there's nothing worse than seeing our babies suffer, especially if the meds won't work. I know too well how it feels to lose them; have lost over 50 rats, as well as other pets, especially over the last ten years since I got into rats back in 2001 when my then young daughter wanted one (and we got two) and we kept on getting more.
I know where you are in wondering how you can stand the pain of losing them. It's the hard part about loving tiny animals like rats whose life-spans are so short compared to other animal companions and who, unfortunately, can have so many various medical issues within that short life-span that have us worrying and often feeling anticipatory grief when we fear losing them.
I only have two older girls right now, 22 months, and with all the grief I've felt over the last ten years, especially in some instances where I'd lose three or four in a month (when I once had 23 rats at one time), I have also wondered about taking a hiatus when my current two go, just because it really is very hard on the heart. That said, I've not been without rats for ten years now. I know it would feel like a big empty hole without them. So I don't know what my decision will be when I get down to one; whether to let the last one go on by themselves and then take a break for a few years, or whether I'll be impulsive and go get two more babies again to keep the remaining one company.
I guess that I always weigh how happy they are to have good, loved lives (and how happy them make me) against the hard part of losing them (although my belief is that we never lose them; they just change form, but their love is always with us and they always feel our love). I had one lady tell me to approach my grief proactively, focusing on how loved my rats are and the difference I can make for rat lives in making their lives good/happy/secure. But that's easier said than done, I know. I can see and understand both sides of that particular fence.
Perhaps you may take a hiatus for a while when your others do eventually go. Or you may decide that you find it too hard to care for tiny animals with short life spans, and if that's your decision, that's fine too. Or, you may decide that, as much as it hurts, that you can't stand being without them. I guess it's a balance between all the joy against the hard times; but love of any kind always comes with grief - we can't have one without the other. I am trying to teach myself to deal with the grief differently; yes, there will always be grief when they need to transition, but to make myself focus on all the fun times and the difference I made for their lives by loving them, and them me. They were pet store rats who, sadly, sell rats as feeders, the only place I could find two baby rats when one of my last older girls passed away, leaving her middle-aged sister behind. I always remind myself that if I hadn't adopted my two, who knows where they would have ended up? Given how much I spoil my rats (bike rides in a cage mounted behind my bicycle seat so that we can take summer jaunts together, trips to the ice cream store, car rides in the car-car cage, etc.), I don't know if anyone else would have adopted them who would have treated them as well.
So, I will cry when each one goes; one has PT right now and I'm doing my best to treat it with bromo (vet gave me another 2 weeks worth today), and I'll cry when her sister goes. But I will also be able to say through my tears 'I'm so glad to have known you, loved you, been loved by you, and to know that you had a safe, happy life while you were with me'. That will give me the strength to deal with the grief. I'm a big suck; maybe I'll take a hiatus, but knowing me, I'll go into rat withdrawal and go get two more babies again.
You need to do what you feel is best for you too. Do you have a good support network at home? You have good people here who can help with these sad feelings. We've all been there. I know how much your heart hurts over Jasper, but if you can reflect on how happy his life was with you - you wouldn't have had it any other way - does that somehow lessen the grief at least a little bit to know that you made big difference in his life and that he was so loved and cared for in a world that is often far too indifferent to little ratties? I hope your heart can find solace in knowing how good a rattie parent you were to him, and how he'll always love you for that. Please heal soon.