Firstly, how you feel is normal. It doesn't want to feel normal, and it's not a normal situation, but your feelings are valid and important, irrespective of the fact you may feel you don't deserve to feel sad or remorseful - you ARE allowed.
Secondly, you both had very valid reasons for feeling a third child wasn't part of the plan you both had. Although at the moment, it feels overwhelmingly like you should have 'made do' or 'it would have been ok' (which may be true), the fact remains that as you said yourself, you both had joint and individual reasons as to why you chose the option you did. It doesn't lessen the grief or sadness at the moment - those things are very visceral for you, but they are worth bearing in mind for the future.
As trite as it sounds, and like [b]thisisnow[/b] mentioned, you both made the decision with the tools you had at your disposal at the time. And there is nothing wrong with that - therefore, you must not torture yourself with the thoughts of what could have been.
I had to have an abortion nearly 4 months ago. I didn't truly feel like I was out of the fog until earlier this month. We had a lot of reasons - some of which were joint, most of which were individual and we did an awful lot of talking (some arguing) and sought independent help from friends, family and in my DP case, a therapist. If you had your abortion with the Marie Stopes, there is aftercare available for you from the counselling services which I strongly recommend you consider - although I'd suggest giving it an couple more weeks before booking to allow yourself to heal physically.
There is a very good selection of post-abortion grief articles on the internet which can explore in greater depth how to heal after such a trauma - some of these were invaluable to me after I had mine - and it also helped my DP explore some of his feelings. We spent a while unable to communicate with each other and dealt with our grief in very, very different ways, but ultimately we came together and keeping healthy communication during this time is crucial. For that reason, talking to other people will help solidify some of your thoughts and feelings, and enable you to rationalise your decision.
It does get better - it never goes away, and nor should it - but the pain eases. Everyone told me life would go back to normal, and it does to an extent - but I always say it's a little different normal. Our child would have been my first (DP's second), so I feel very empty some days, but I do know I made the right decision, even if I wish I hadn't had to make it. Please look after yourself - give yourself space and time to heal, come and talk to us on this board, write down how you're feeling somewhere private - whatever works for you.