Believe in yourself, there is no limit to what you can achieve. A lot of people do doubt themselves. Let me tell you, if you ever for one day doubt yourself then success will not attract you. Success is a thing of the mind. Once you think it, it starts to manifest. Do not doubt yourself anymore. Believe you can achieve great things. It's only then you see yourself achieve exploits.
Doubt in a simple context means to lack confidence in something and, according to Celine Dion "It can close you in, build the walls within and let fear control you". Believe me once you are captivated by the spirit of fear, the less you can stand for what you want.
Doubt is a time waster, because you'll keep pondering on the matters ( i can do it, and I can't do it). It will also do you no good rather it'll make you lose great opportunities, the only one thing you need to consider is the risk reward ratio.
It is pertinent to know that doubt is a killer, you just have to know who you are and what you stand for. Always aim high, work hard and care deeply about what you believe in, when you stumble keep faith, don't give up. And when you are knocked down, get right back up, and never listen to anyone who says you can't or shouldn't go on.
My dear, life is very interesting and you can't afford to miss it. At the end, some of your greatest pain, becomes your greatest strengths if you believe in yourself, have dedication and that never quit attitude.
Remember champion keeps playing until they get it right.