What you're describing is called "the pull out method" or "the withdrawal method," and doctors and just about everyone else have GREATLY debated the effectiveness.
Anecdotally, I could tell you that I know people it's worked for. Personally, I'm going to tell you it's a tricky birth control method in that you really need to do it perfectly for it to work and even then - you're playing with fire, so to speak. But I guess this could be true of any other birth control method, right?
According to Planned Parenthood, "For every 100 women who use the pull out method perfectly, 4 will get pregnant." Those aren't terrible odds, but they're not 100% either.
So to answer your question, the withdrawal method, when practiced without flaw is effective against pregnancy 96% of the time, according to Planned Parenthood's statistics.
If you'd like more information on birth control options I recommend https://bedsider.org where you'll find all kinds of articles and cool features on everything from condoms, the patch and yes, withdrawal to help you see all of your options for preventing pregnancy and how to obtain them.
If you decide to go with a method that takes less willpower talk to your OB or general physician about what they can offer you.