You're trying to talk yourself out of being depressed. Are you talking your husband out of that virus? No? Why not? Because you accept that his illness is something that is happening TO him, not something he is doing to himself. But with mental health, we really put pressure on the sufferer saying they should just snap out of it, be grateful for what they have, blah blah. What you have is no different in "value" than what your husband has. Plus you are exhausted with 3 kids. You speak of your responsibility to take care of your family, but you don't mention anyone's responsibility to take care of you.
You are also googling various causes because you're vacillating between being afraid and somehow "hoping" there's a diagnosis for it that makes it worth your time and others' attention ("need an MRI"). On some level, you aren't sure you "have some condition" that merits attention or expense or time away from the kids. So there's a fundamental dismissive attitude in this country toward mental health - as if we don't deserve treatment unless we are horribly ill. We think a schizophrenic person deserves care, but we don't think a mom with anxiety does. That's like saying, "You can only go to the physician if you have cancer, but not if you have acid reflux."
May I suggest that you not try to diagnose yourself? Just list your symptoms but don't decide if it's a family-history-based illness or postpartum depression or anxiety. Just make a list - anxious feeling, head zaps, wanting to curl up with the iPad, etc. Then go over this with a therapist. Maybe you can benefit from medication, maybe talk therapy, maybe some relaxation techniques (which need to be taught - don't just tell yourself to "Relax, for crying out loud"), and maybe other things or a combination. Don't worry about the treatment or the diagnosis right now. Just decide that you will go talk to someone and lay out the symptoms without having to go in there with a ready-made diagnosis.
Reach out, ask someone to watch your kids, and go see someone.
Yes, these years should be savored, but you have a problem that needs to be addressed. You don't blame your husband for not savoring the children while he's sick, do you? But you put that enormous pressure on yourself.
I went to therapy and actually found that what was hampering me was not what I thought it was. So ask your doctor for a referral to someone who accepts your insurance, call a friend or hire a sitter or find someone with evening or Saturday hours, and GO.
You deserve this. If it helps motivate you because you still aren't sure of that, consider that your children are entitled to a mom who is in the best possible condition to care for them. I grew up with a mother who had periods of depression and almost emotional paralysis. I wish she had had the courage and the support to give voice to her symptoms and seek help.