Read books by experts in the field to understand the why's behind the behaviors of children adopted out of foster care, which will give you a foundation for finding solutions. Parenting the Hurt Child: Helping Adoptive Families Heal and Grow by Gregory Keck and Regina Kupecky is a good one to start with.
Find foster adoption mentors and supports. Mine is the former director of the agency we adopted our son through; she and her husband have 5 bio children, 8 adopted children, and have fostered 80 + children. They've seen it all, are surprised by nothing, are non-judgmental because they've "been there," and have had great experience and insight to share with me when I've encountered situations I didn't know how to handle.
Have age-appropriate, honest and open dialogue with your child. We adopted our son when he was 7, and I've always let him know that if he ever had any questions or concerns he could tell me and if I didn't have the answers, solutions, guidance, etc., he needed, I'd find someone who did.
Find an adoption competent therapist and attend sessions with your child. Find an adoption support group and if you can't find one, start one. It doesn't have to be big. Mine started with two other moms, we've added a 4th, and we have helped each other through many a struggle. (Dads are included, too, in our family-oriented gatherings and in whatever other way they'd like to participate.)
Have a spiritual life. Doesn't have to be organized in any way, but believing in something bigger than myself to call on for help when I'm overwhelmed has kept me from going over the edge more times than I can remember.
Work on yourself. In my experience, if I'm being triggered by my child's behavior, likely it's touched on something within me I need to get to the root of and address, with the help of all the above supports.
Practice gratitude. In the low moments find something to be grateful about. The sky. The air you are breathing. Your cup of coffee. Having a toilet. It's impossible to be negative and grateful at the same time.
Most importantly, take it one day at a time, remember parenting is a marathon, not a sprint, and put your own oxygen mask on first. Taking care of you by getting enough rest, exercise, healthy food, quiet time, and relaxation will help you be the parent your child needs and deserves. Being my son's mom is the best gift of my life. I wish the same for you and your future foster adoptive child!