This is true that starting a business is not easy and many people fear go through this space however after some point of time things get better but initially it's not that much easy. and as you mentioned rightly that it's important to be invest the profits time to time for the better growth of the business and it will also help the business to be sustainable in long term.
Hello dear @alokkumar121, fear appears in everything that is new to us, because we start thinking about the setbacks we may have and not in what we can achieve if we take the risk. Apart from that, after you start you must have an excellent management of resources, to be sustainable as you mention.
See you later brother and have a great start of the week.