Welcome to our forum.
There is a Book and Website Recommendations thread at:
Some specific book suggestions:
Polyamory: roadmaps for the clueless and hopeful ... by Anthony Ravenscroft.
Polyamory: the new love without limits ... by Deborah Anapol.
The Ethical Slut ... by Dossie Easton and Catherine Liszt.
Sex At Dawn ... by Christopher Ryan and Cacilda Jethá.
The Monogamy Myth ... by Peggy Vaughan.
Love in Abundance
Open ... by Jenny Block.
Opening Up: creating and sustaining open relationships ... by Tristan Taormino.
The only thing I can recommend for D is time and patience. You have to consider the possibility that D will always be like this, and decide if that's something you can live with.
"How do you find the time for both yourself and your partners?"
I think in a lot of cases you have to keep a calendar.
"How do you make sure you take good care of yourself, while being very much in love?"
Awareness is your best friend. Be aware of the need for balance between yourself and others, and how you're doing on that balance.
"On which beliefs do you base your polyamorous life style?"
I just think it's okay to share partners. No complicated beliefs there.
Hope some of this helps.