The guilt is I guess normal. You'd probably feel the same way about a human being you've loved.
I have a different approach, because I'm used to most people I love and care about leaving me. Whether they actually ever cared about me or were just faking it to get whatever they wanted (that has been true in most cases). But then I've always had someone new come in their place, even if they later leave
If I let myself feel guilty about loving the next person, I'd just be robbing myself, because even if it's for a day or year, they're making me feel good too.
With the dogs I know that their love was genuine, and I haven't truly felt them be gone. They're definitely a part of me still, in how I behave and love. And I feel them like a comforting, protective presence. They send me signs when I need them.
Any guilt I feel just stems from not doing more. Not being more present (school and stuff got in the way), scolding them for something or the other (mischievous lot, all of mine), not giving them more (material comforts, though I've always gone above and beyond budget etc).
And the only negativity around getting a new dog was the same as being around a new human - trying to close off my heart a bit so it wouldn't hurt so bad this time. Of course that's a stupid thought where animals/birds are concerned because they're here for such a short time and they love with their whole hearts so deserve that back too. And they'll take it from you whether you want to or not
One thing is 100% sure. That our animals/birds would want us to always love and be loved wholeheartedly. Even if they were jealous and possessive AHs when they were with us
I don't think my first would have sent us the next one otherwise. Or the one they sent to us after that. Or the Pup. God knows what someone else might have done to him, but we're what he needs and he is what we needed, even if it results in some blood being drawn every other day!
It is also 100% okay to let go if you realise that a pairing is not a good fit. Our dogs literally chose us. The second one didn't and he was very angry about being sent to us (his owner basically lied and kicked him out too young), so it took us all a lot of time and patience to become a family. Maybe if we hadn't been grieving the first guy and so desperate for another, we couldn't have done it. Same with the Pup. Love at first sight but almost every day is a challenge sometimes. Difference is that we are three people, so there's always someone to take care of him or a specific need of his when others can't. And I also got a job just in time otherwise we would have had to give him up. I honestly didn't even want to bring a pup home because I was severely depressed and knew I couldn't take care of one, and I knew my parents wouldn't. They do a few things, but there are so many things that I have to pick up the slack on.
So it's okay either way. Have a pet, don't have a pet, rehome a pet because you can't care for them the way you would like to/they need, that's actually pretty selfless. If anyone gives you shite for it, they're the crappy ones.
And also, allergies can develop suddenly due to illnesses or stress or reactions to vaccines etc (mum's Caeliac developed as an adult that's why it couldn't be diagnosed for more than a decade - docs here were used to it being a paediatric disease), you had no way of knowing before you got Inky.
keeping fingers and toes crossed for you. Hope you are feeling well enough to garden thankfully the weather is behaving better- The slugs/snails did for my lettuce overnight- went searching today and flung the ones I found into the field next door with the dogs tennis ball launcher
… sorry to hear you are going through a few things at the moment- hoping all will be well soon
- later on it got a bit easier to see them with the naked eye- 
