It's impossible to say without knowing a lot more details, but one thing that may be contributing to your troubles is the USDA loan. These generally don't require a down payment, often take a long time to close and have a questionable reputation to some people.
A cash buyer will always have the upper hand in competition with any loan just because they aren't contingent upon getting final approval.
FHA with 3.5% down may give you a little better shot if you can scape it together, at least those typically can close in 30 to 45 days.
Another issue you're facing is the less expensive the home, the more competition there will be for it. Other first time buyers and investors. There isn't much you can do about this, but it's going to continue to be a factor.
If your lender can provide any assurances with the pre-approval letter that they can close in a timely manner, that may help. Offering the seller anything that they want in the deal can help.
My guess is you'll have to keep on trying until you finally win one. I'm sorry for you struggles but many others are in the same situation.