HUGS
My family lives in a different country, so I have to fly over regularly to keep in touch. Over the years I have lost the odd piggy while away (one of them a year ago; she simply didn't wake up from an after dinner nap) or found them seriously ill and dying upon my return.
My 50th birthday was spent standing by a canal in Amsterdam on the phone to my vet to give permission for a make or break emergency operation of one of my guinea pigs and then nervously waiting to hear back. It was my greatest birthday present that Ceri did make it through against the odds!
It is a risk I have to take every time I leave; sometimes a piggy is already very frail or somebody who is not so experienced with guinea pigs is unable to spot the often very subtle signs of illness in aminals they are not familiar with.

I do however NOT feel guilty about leaving my piggies! I love my family as much as my pets; they are not in the same place.
But it also goes the other way round: it was heart-breaking for me having to say a forever goodbye to my dying dad knowing that I would not see him alive again; I flooded the plane with my tears on the journey home!
I try my best to find reliable carers (it often is a trial and error process) and I have a boarding place with somebody very experienced for my oldest and frailest of piggies that need regular medication - but even that can never be a 100% fail-safe guarantee.
Of course, it hurts badly when I lose one of them; and especially if they are as young or younger than yours! There is always a measure of regret that they were not able to realise their full potential - but that can happen to any piggy irrespective of whether you are away or not, and sometimes even rushing them to the vets asap cannot save them; I have got several t-shirts with that logo, too!
Stuff happens, like my 3 year old Myfina going downhill over Christmas Day when the remaining local out-of-hours service was totally snowed under with emergencies; not that they would have been able to save her!
If I have learned one thing over the decades of having guinea pigs, then it is to treasure every day you have them in your life. You cannot control or choose how long they live and what they die of, as little as you can do that with family and friends. We are not god! Not every human is allowed to live to an old age, and not every guinea pig will ever do.
But in the overall scheme of things it is also weighed up by those piggies that live to a grand old age and longer than average. I rejoice when this happens. It is a special boon, not something that is my own achievement, as little as an early loss to illness is my fault!
The most important point is to make the best of the time you have got and to love your pets every day, so you can keep them in your memory with the knowledge that you have not failed them, no matter how and when they have died. Concentrate on what you can do and do not fixate on what is out of your control and what you cannot change!
You have clearly loved your girl very much; she would have known that, and she was loved when she died. Don't lose sight of that and don't let the end overshadow all the good times and all the love you have shared!
You may want to start a diary in which you write down all your memories, the little and the big ones, as they come to your mind. Also write about your feelings. If you do it for some time, you will find two things - that there is more of your girl left than you think, and because it is written down, you will never lose it. And secondly that over times as you cherish the good things you have shared together that your guilt will be counterbalanced by the happiness that connects you, too!
PS: If you continue to struggle, please consider speaking to the free UK Blue Cross pet bereavement line, which is manned by specially trained confidential volunteers. The best thing you can do is to talk about your feelings in detail. You can't hear often enough that you should feel sad about your loss, but not feel guilty about what you didn't cause and could not prevent. All of the forum members that have used it (many of whom suffering from the same persistent feelings of guilt or failure) have found that it has really helped them come to terms with what has happened.
SupportLine - Problems: Pet Bereavement: Advice, support and information