I don’t think anyone has meant it harshly if they’ve used the word ‘selfish.’ In some ways you could say any of us who have children are ‘selfish’ in that we’re fulfilling a desire.
I also think it’s very natural and common that once the youngest child in the family is reaching an age of independence, once they’re at school, many mums feel an urge to ‘do it again’. But that’s very different from genuinely believing another child is the best thing for your family.
FWIW we both always planned 3, I popped them out in quick succession which was blooming hard from a financial and practical viewpoint but meant as a family we went through each stage together as a unit. I had short maternity leaves (this was over 20 years ago) and it meant I never felt I’d completely left my old life behind IYSWIM. Even so, when dc 3 started school I remember feeling a bit of a pang that this was it, no more pre school kiddies. If I hadn’t had my busy work life no doubt I’d have felt it even more acutely. But I suspect that feeling is there no matter how many kids you have... there will always be a youngest and that youngest one will eventually get to the point of starting school and not needing so much constant attention.
They really do need you in different ways though... OP as you have one just entering teenage years you’ll discover that. And it’s no less needy than a toddler, just different needs.
Sorry for your miscarriage. I suspect that may play a part in how you feel, but as someone else said, that loss will always be there, you can’t replace that child, you could have another 3 children but you’d still have that loss 
One positive thing though ... those pangs really do fade. I remember seeing a friend with a newborn just as my youngest was starting school and I felt a really pang or ‘oh my goodness, I could have another one!’
But within 6 months the feeling had completely gone and id actually have been really distressed if I’d had an accidental pregnancy. Once my youngest turned 5 we’d really moved into a lovely phase as a family, no more night waking, crying, tantrums etc. The friend with who I’d seen with her newborn also had older children the same ages as mine and honestly once that baby reached crawling stage and I saw the harsh reality of combining a mobile baby with older children I actually felt a little sorry for her - it became really hard work