There are many people who have become successful without diplomas. Thomas Edison is one example: only had a 3rd grade education, yet was one of the most prolific inventors in US history. In fact, if memory serves, he coined two phrases: "I've not failed, I've just found 1000 ways that didn't work" (I believe it was his inventing the light bulb that required 1000 tries to get right) and "genius is 1% inspiration and 99% perspiration." Michael Dell dropped out of college to start up Dell Computers and look where he is today.
Can everyone be brilliant? No. Can everyone be a Michael Dell? No. In fact, I believe it was a 1976 survey by the Canadian government of over 10.000 Canadian businesses that found the following: 2% were leaders; 8% had innate qualities to become leaders, but had to be trained and given opportunities to lead; the other 90% just 'showed up' and collected a paycheck.
The best bet is always to do your best, but start early in finding out your strengths (talents) and weaknesses. That way, you'll know how to guide your educational experience to play to your strengths and build up those areas you need to succeed in that field. Take tests like a Myers-Briggs Interest Inventory test, take other related psychological and personality profile tests, as they are anywhere from 90-95% accurate and will help you better understand what fields you could succeed in, as well as which ones your personality is also best suited for. This will also better help one define one's goals accordingly. The best advice is to find study and do what you're best at (most passionate for), then find a job doing that.
That done, it's a good idea to look at job wanted ads to find out what level of education is absolutely necessary in the field and country you live in. For example, here in the US, if you want to work in computer tech support, as far as you really need to go is to get a 2-year degree (A.S.) and appropriate certification for that field (good 2-year programs nearly make certification a foregone conclusion--if one puts in the required work). Basically, the maximum education required for most jobs is a bachelors (4-year degree). Here, employers in the US (since about 2000) consistently discriminate against those with too much education and/or experience in most cases. You'll even find many job wanted ads that only require a high school diploma, but with a certain amount of experience (it's possible to get the training in high schools these days, but where the devil they expect you to get the experience, I do not know). Only get very advanced degrees (Masters, Doctorate) for fields that absolutely require it. Another necessity these days is to form lasting friendships/genuine relationships with other people. You never know when those 'contacts' will come in handy, especially for a job. However, it should be a mutually beneficial relationship; don't expect to be able to only solicit someone's help when you need it, write them a thank you note, and that's it. It may be quite an investment in time, but it may pay off in ways you can't predict.
Regardless of how much education you get, keep learning new things, things you perhaps would like to do but have little experience or talent in, for you may find other skills or interests that prove valuable to your future. Always realize every person has limitations: it's rare one person is good at everything. Accept that there may be things you'll never be good at. Also realize that no matter how hard you try, you'll never know nor master all there is. The best way I heard this explained was by a Spanish professor at Texas A&M that was very realistic about his educational experience. He said, "when I got my Bachelors, I thought I knew everything; when I got my Masters, I began to doubt it; when I got my Ph.d, I knew I knew absolutely nothing."
The most important thing: never let others define 'success' for you. If you were to make an 'average' living (not poor nor rich), but had enough money to cover your expenses and still have a little left over for enjoying things, be glad you have it. It's better to make (as an example) $35,000, love your job and life than to make $125,000 or more, be stressed out and miserable. Money does NOT make the man.