instead of thinking your writing sucks you should think - I don't know anything at all about the quality of my writing from this.
That's why the onus is on the aspiring writer to research and learn about their craft.
There are plenty of ways to get critical feedback on a work - everything from crit groups to editors, as mentioned above. All are readily accessible - even more so now in this internet age.
And agents have written books to try and inform writers about what they want, what they expect, and how the industry works - Carol Blake and Donald Maas come to mind immediately. I'm sure there are others.
If you go to writing conferences, agents will often be available to answer questions.
It's not that the writing industry is more unfair than any other discipline in life - it is simply that the expectations of success are often too high.
It's all too easy to think that stringing words together = storytelling, but that's simply not true. Research will reveal all sorts of useful tools. Crits and editors can help show problems.
Agents may be gatekeepers of a sort, but there's plenty of opportunity to prepare before meeting them.
2c