Its human nature to feel this way. We all know the saying (or the 80's tune for you big hair folks), "Once Bitten, Twice Shy". Or what about, "All good things must come to an end."? Its how we react to almost everything that affects our well being and general safety. Its a subconscious reaction at the gut level.
Thankfully, unless you provide very poor service, most clients will eventually pay you. However, it may take you awhile to collect your money and you may have to settle for less than the originally billed amount. You have the option of adding late fees to your invoices, but keep in mind, the more time you spend trying to collect a past due invoice, the less time you have to devote to paying customers. Also, if you need to engage an attorney or collection agency, you will in all likelihood, not see the full amount of your invoice because of their fees.
Everybody should know that under current tax law you can sell your primary residence and any capital gain up to $250,000 ($500,000 if married) is tax free. Wow... is that powerful!... and why aren't more investors taking advantage if that "loop hole"?