Firstly, I'm sorry it has taken so long for you to be prescribed medication. Take it and be consistent. It will help massively.
Secondly, it will get better. You have done nothing wrong and have nothing to feel bad about. Life is full of difficult decisions and all you can do is what seems best in that moment. Trust yourself that you did what you needed to do in the moment and whatever reasons you had, they were completely valid. I can tell from your post that you are a reflective and conscientious person who would not have rushed into a decision otherwise.
That tiny embryo had no consciousness, no real development - the only person who was affected by this decision was yourself. Your other children will not miss another sibling that they never had. So many things can go wrong in a pregnancy and who knows what would have happened if you had continued the pregnancy in the state you were in, along with there being no guarantees in it even ending in a healthy baby. A pregnancy does not always end in a baby for many reasons. All the babies you could have had in your life time, infinite possibilities, who knows what could have been - focus on what is definite and true: your existing children and your other loved ones.
It sounds like you have given yourself the hardest time imaginable; try and practice kindness to yourself. Affirmations every day help: Things are hard at the moment but they will get better.
Every day I am healing and growing stronger.
I made the best choice I could for myself in that moment.
Your mind hears what you tell it. Try and talk positively to yourself.
Focus on living your life (hard as it may feel). Remember activities you have enjoyed in the past and things you couldn't do if you had a baby. Go through the motions and find the little sparks of joy in the simple things.
Get blood tests and your thyroid checked. A sudden drop in progesterone at the end of a pregnancy can also trigger thyroid issues which can cause or exacerbate anxiety and depression. Take magnesium glycinate if you're having trouble sleeping. If you're struggling to eat then try and have little snacks (pre-prepared) throughout the day.
But overall keep on going through the motions of life and you will get there. It will not always hurt like this I promise. If you decide you want a baby, try and have one. If you don't then that's fine too.
Good luck and strength to you.