Hi Tammy,
Sorry to hear about your client troubles, it is always frustrating to work and then not get paid. I am happy to hear that you're not letting this bad apple spoil it for you, great attitude!
One point I'd like to make is that you should give yourself credit for getting this client to finally sign a contract, which is a really important point when protecting you and your business. As for it being intimidating asking for a retainer upfront, remember that as long as the client has signed the contract stipulating that as a term, you shouldn't feel bad about enforcing that clause.
They have agreed to those terms, don't let them wheedle you down!
Best of luck with your business, I hope your next client is an amazing and ideal one!