As you may know, the VA can take months or even years to approve a disability benefit claim. For that reason, they usually award benefits retroactively for the time period between when a veteran applied and when they were approved.
For example, if you applied for benefits in March 2019 but weren’t approved until September 2019, you would be entitled to six months of backpay for that span, in addition to the monthly payments you would receive moving forward.
Unfortunately, unless you were discharged from the service or exposed to Agent Orange, you are unlikely to recover retroactive awards for the time period prior to your benefits application (regardless of when you were diagnosed with your condition).