Yeah it is correct that you gotta have a strong belief to succeed but the question is believe in what? If you are pursuing greatness, if you are pursuing a huge goal you can't be forever believing. Sometimes it is hard to believe anymore especially if a great challenge is challenging you.
So if you can't believe anymore then you should stop believing.
Stop believing your doubters and haters. Stop believing them, they were just jealous because you are pursuing something. Never even listen to what they say nor give them a single second. Just focus on your goals and do whatever it takes to make it happen. Haters and doubters were your confused admirers. They were always following you, hating you, pulling you down because they know you can succeed and they don't have the qualities that you have. Stop believing them even if they sound real, remember that snakes are good in deceiving. Stop believing things that are not empowering you. If they say something to you, just forget it, laugh at it and keep moving forward.
Stop believing that you are weak. Your belief is the measurement of your strength. There will be times where you are the one who is doubting yourself, you will tell to yourself that you are weak and you can't do it anymore, never listen to the weak voices that are whispering your ears. You are strong, you can find the strength how to get things done. Trust yourself and everything will fall into its designated place.
Stop believing that you can't do it. Of course you can do it, everything is possible and there is no way you can't accomplish something. It doesn't matter if you don't know how are you going to do it , what matters is you will do it. You already started it so it only means you can also finish it. Every beginning has an ending, never stop in the middle, push further and reach the finish line.
Sometimes it is really hard to keep the faith especially if the situation is really hard. Somewhere along the journey, you would love to quit and go back. But what is the point in going back? if you can't believe is something then you should not believe the things that are stopping you from believing.