The right therapist WILL be able to handle poly and non-monogamy. A well-experienced therapist who has his or her head screwed on straight will take such things in stride and will not judge you. When I talked about it to my therapist she smiled and said that there are many kinds of relationships besides monogamy and that mine sounded loving and healthy. Then we started exploring.
Ask your girlfriend to call therapists in your area and find some that have had experience counselling poly couples. Ask if they offer a "get-acquainted" session -- a short visit at reduced rates to allow both patient and therapist to see if the personality fit is good. Visit a few and choose the best one.
I think therapy is a very good idea.
As far as your situation goes, I can only suggest that you and your GF take a break from outside romances, or put them in holding patterns, until you can find out how to move out of the jealousy disaster zone. I don't know what else to say.