The shelter I worked at was very similar to the one Bunnity described. Kill shelter, but with a guarantee that 100% of adoptable animals are allowed to stay until they're adopted. Pretty large shelter with 7 thousand dogs and cats coming and going every year plus all types of other companion animals and wildlife. 150 staff members total, with 4-5 full time vets, 7-8 vet techs, and 17 animal control officers (who were of the nice compassionate kind). I started as a volunter, and I think after a couple hundred hours I was hired over the summer as a relief technician.
To the OP: If the volunteer opportunity was anything at all like the one I was offered, TAKE IT! or at least give it a week or two before you give up on it.
It was the best experience EVER! I almost never had to clean kennels or feed the animals. Almost everything I did was 100% vet med related. I started off learning how to restrain animals and medicate orally and ophthalmically (?), and a week later learned how to give fluids subQ, and a week or two after that learned how to draw blood. Sooner or later I learned how to prep the animals for surgery (everything from knocking the animal down to intubating, to shaving, to post-op recovery). The awesome thing is, there are so many animals that NEED the help, so you get really good at new procedures super fast.
Knowing that I was interested in becoming a vet, the vets there really took it upon themselves to involve me anytime something interesting was going on.
As for the depressing part of the work... I would really just try it out for a while. The whole atmosphere is very sad at first (esp when you see rows and rows of kennels with dogs going crazy, and cats crammed into small kennels), but that passes very quickly. As aweful as it sound, most of the animals are in better condition at the shelter than where they were.
Even euthanasia gets easier for the most part (given that the shelter has a good euthanasia practice using Buthanasia). It's a pretty peaceful way for animals to go for many of them. For the dogs that like being around people, they actually really enjoy those last few moments because they're so excited to get out of their kennel and get attention. They don't even see it coming, and they're knocked unconscious within seconds with an IV injection. We did IP injections for cats, and many of the cats 'fell asleep' purring as I pet them after their injections. It can feel really disturbing that these seemingly happy animals are getting killed, but I think that's more of a selfish human feeling, if you know what I mean. It's only sad because people have this abstract way of thinking about "what could have been". Dogs and cats don't see things that way.
There are some things that never gets easier though... To this day, I still hate seeing the barrels and barrels of dead animal all piled on top of each other. Abuse cases really disturbs me. There are animals who are petrified, lost all hope, or are super agitated in confinement. Those are very sad.
For some reason, I've developed a coping mechanism where I feel very composed and almost stoic while I'm dealing with something really sad in the shelter. I rarely ever cried on the job. But there were definitely days where I would burst into tears the second I got into my car, and I would cry all throughout my 40 min drive home. I also broke down while giving a talk about the inner workings of a shelter. It CAN be very taxing, esp when you tend to fall in love with the animals that you know is likely to be put to sleep.
But despite these things though, I see my shelter experience as overall an extraordinarily positive one. I learned soooo much. Gained soooo many skills. And after 35 foster animals, ended up adopting 2 of the most fabulous cats in the world! And I found a lot of the work I did very rewarding.
Just remember that any little thing you do for any one animal at the shelter will make a difference. It's very overwhelming to see all of the sadness at once, but you can personally make it that much better for each and every animal that you take the time to help.
