I'd just tell her. If you let her know how much you appreciate all the work she's done with you over the past 6 or 7 years and even give her a few examples of areas you feel she's helped you with, she won't be angry. She might be a little sad to see you go, but if she's as compassionate and caring as you feel she is, she'll probably be glad that you have grown but feel the need for a new treatment angle. AND if you're right about her feeling that she is struggling in the area of helping you, she might actually be relieved that you see the need to move on and continue to grow.

As for finding a DBT therapist, ask the staff at the PHP program. They'll know who in your area is trained and working effectively with DBT techniques. Good luck and it's good to hear that you are feeling so positive about the program you were in!