Can I get Invisalign if I have a filling that my dentist said is “really deep”?

Hey there! Having a "really deep" filling could be problematic because, like your dentist mentioned, it's close to the nerve. Unlike other nerves in the body, teeth can only recognize pain so when something starts to encroach on it you'll feel pain there. In your situation, I think it's important to have all your teeth and gums totally healthy before starting any orthodontic treatment. Once that happens, you should be 100% fine to begin Invisalign treatment. Invisalign is very different from SDC, even though they look similar. My suggestion is to find a local orthodontist to have an open conversation with about your past experience in treatment and how you want your smile to change. They'll give you the best and safest treatment to work WITH you to achieve your goals. PS- they may even offer a discount because you're an unhappy ex-client of a DIY clear aligner company!
Smile Big, Dr. Brice