@Alwaysohard I think it's very brave of you to recognise and admit your feelings here - you've clearly got your hands very full with 4 children who are adopted, with all that entails, and birth children.
Love comes in very many forms, the warm, fuzzy, idealistic feelings (that birth parents have for their children despite not being able to care for them), the active caring, watching for, meeting their needs kind of love, the companionship and care we have for friends, the desire we have for our partners etc etc. Duty is another kind of love, we do what we need to do because we care and made a commitment to our kids.
I'm not surprised taking your birth children away felt like a relief, even just lessening the number of people making demands on your time and attention is a relief of sorts, because you can attend to one set of needs at a time instead of trying to spread yourself too thinly. When you have multiple children, all with challenging behaviours and competing needs it can be hard to find "love" for each of them because you're so busy dealing with the present issue you can't take the headspace to find connection and connection breeds love.
Try not to feel guilty, no-one can be anyone's everything - if that's the goal you're setting yourself in terms of love, it's too high a bar. We need different things from different people at different times, and so do your children. They may need you to do what they need practically at the moment, and that is a kind of love in action, as another poster said. At other times they will need care, compassion, comfort, affection etc etc and that's another kind of love.
I wonder where you get your needs met? Where is your time to connect with family and friends without the stress of children in the mix, where is the time for you to rest and recover from the demands of caring for such a large family, where is your time to do whatever feeds your soul? Without that, how can you possibly offer love to these little people, you literally can't pour from an empty cup - and you sound physically and emotionally depleted.
On a very human level, we connect differently too, I loved my little boy from the moment I met him - he was very easy to love, cute, affectionate and easy going. With my DD it took much longer, she wasn't so easy to love, very demanding, challenging behaviour and at times a hard personality, but love did come as I grew to understand her and her way of being in relationship. But I needed lots of time for my own growth before I got there.
I hope you're doing OK, go gently with yourself, you're doing an incredibly hard thing.