HMO referral miscommunication led to big bill

I have an HMO plan but was seeing this neurologist while I was on my parents PPO. When I gave them my updated insurance info I asked them if I needed a referral cause the only other specialist I see, a dermotologist, told me I needed one to keep going to the Dermo. The neurologist told me I was fine and that I did not need one. I had 1-2 appts with them that were covered and I only paid a 30$ copay. Then they wrote a letter to my insurance to get Botox for migraines approved and told me my insurance approved it. So I went for my first injection and before I got it I asked if I would owe anything and they said no you’re good. So I assumed it was completely covered. I went for the second in august, didn’t pay anything then either. Just got a notice from my insurance that they were only covering part of the bill because there was no referral and I owe 534$ just for the first injection. If I knew it wasn’t covered I would’ve never gotten the second injection until I got a referral. Is there anything I can do to lessen my bill? I asked my insurance if getting a referral now would make a difference and they said no. They said it was my responsibility to make sure the services would be covered and I shouldn’t have trusted my doctors office. I get that- it just really sucks. I’m on food stamps and I’m an unpaid intern therapist. Having a really hard time with money right now, breaks my heart that I’m gonna owe 1,000$ just because I didn’t get a stupid letter sent from my PCP. I’m also just confused because I wasn’t charged for my other apts that I went to without a referral.