I'll have a stab at this, just in case your post gets buried and nobody else answers it.
First of all, whilst your feelings towards this teacher sound completely understandable in the circumstances, and because you've been talking to her for long, if I were you I'd start looking for little ways to distance yourself from her now, because you are so close to leaving school and not having her for support in the future. That should make the wrench when you leave school a little easier to handle.
If you see her on a regular basis, or have a set time each day/week that you see her, maybe you could try to space these out a bit more?
Once you've found somebody supportive it's easy to fall into the trap of depending on them a little too much. It isn't your fault, because you're vulnerable at the moment - it's a natural reaction, but it can still be problematic.
I had a teacher at school who helped me out for 2 years and who was my form tutor for 5 years, but she changed schools and moved a couple of hundred miles away when I was in year 12, which I found really difficult to deal with for a while, which is why I'd encourage you to slowly start preparing yourself now by not seeing this teacher quite so much, and by letting her know why (that you're aware you're leaving soon etc.)
You don't say in your post, but I presume you've been diagnosed, and have some other form of support in place? Whether from your GP, CAMHS, etc. ? If you don't, I'd strongly recommend seeing a doctor and getting some other support in place. Teachers can be excellent listeners, good empathisers, and have some level of training for talking to students, but in most cases professionals such as GPs, psychologists, psychiatrists and counsellors will be much better placed to advise and give support.
When it comes to talking to other people, I understand what you mean when you say it doesn't feel as helpful as talking to this teacher. An understanding with a person, whether it be a teacher, a friend or a doctor, comes with time. When you've found a good listener you don't want to let them go because it feels like nobody will understand you in the same way that they do, but I can promise you that there are lots of other people out there that will do a good job if you give them time and a little of your trust.
I know it can be difficult - I've felt it when my teacher left; when I was transferred from child and adolescent to adult mental health care; when my schools nurse was reassigned - but the truth is that after a few sessions with a new person, provided that they are good at their job (and most of them are) you'll feel just as supported by them as you do now by your teacher (though I wouldn't expect hugs from a psychiatrist
).
Hope that helped, even if it was a little rambling and long.