I am a Catholic who believes in the existence of evolution.
Not very firmly, since you go on to say:
However, people saying that evolution created humans is kind of an over-statement of evolution.
Actually it's an understatement. Humans appear very late in natural history. Humans are closely related to apes as I'm sure you know. Similar evolutionary forces acting on apes have acted on humans. There are many humanoid (hominid) forms that preceded us. Their fossils form part of the ever growing body of evidence of human evolution. So do their cultural artifacts, such as tools, or, in the case of Neanderthals, their burial sites and artwork.
For a particle to turn into a human there would have been some reason for this to have happened.
Yes, the union of two single celled organisms, the human egg and sperm, are particles that come together for the purpose of sexual reproduction, which is an evolutionary-favored means of speciation and survival.
Evolution occurs because of environmental reasons.
That's one of many sources of natural selection. Evolution occurs because of genetic mutation over repeated episodes until a trait is acquired that favors survival under some change or "niche" condition.
Also, evolution is not adaption of species, it is removing the factor of the non-fittest and allowing the ones that have the ability to reproduce and later on genetically improvise their current structure, however to an extent.
This is precisely how adaptation occurs -- by natural selection.
If you could explain how humans had evolved from a particle to what we are today without any broad sense of the idea, it could be a good idea to look at
Humans did not evolve from a particle unless you are referring to sperm and/or egg "particles". Humans evolved from proto-humans, and many of our gradual changes in form from, say Ardipithecus or Homo Erectus, until the present form of Homo Sapiens Sapiens, have been fossilized and preserved as evidence that we did in fact actually evolve no matter how much that conflicts with your religious ideology.
but simply because evolution can explain how a turtle evolved to grow a long neck from a short one over thousands of years doesn't supply sufficient evidence to the idea that humans evolved from small particles.
You need only understand the turtles of Galapagos to understand why they evolved long and short necks. It's quite fascinating, since the plants they were reaching for co-evolved. Both the food and the feeder went through natural selection, and the process varied on separate islands, causing them to diverge.
You are missing the more significant point about Galapagos. The species that occur there were created there, long after birds were inhabiting South America. For examples, the finches were created by adaptive radiation. It's a process of splitting into many forms when multiple niches are available for exploitation. They had no competition, so they ended up over populated and competing among themselves. They adapted -- evolved -- to eat just about every form of food a bird can survive on. And the iguanas developed the ability to dive into sea water and collected sea weed, and then, of course, to digest sea weed.
Yes, Galapagos is the key to understanding evolution by natural selection. And evolution is necessary to explain all the weird early human and humanoid skulls and skeletons that preceded us millions of years ago.
And all of this is acceptable to the Catholic Church. So you're at liberty to study it, learn it and to accept it.