I have seen two main types of jealousy in relationships. One is envy where you want what your partner has. The other is insecurity, where you fear the loss of something like security or that special feeling he has for you. It sounds like you are feeling the latter.
It is ok to feel jealous. But the best use for jealousy is to help you understand yourself and what you are feeling. It sounds like you need more reassurances from Matt that he still loves you in that way you want to be loved.
One thing you are also dealing with is Matt's new relationship enegery (NRE) for Sara. That is newness of a relationship that makes you feel extra good and want to talk about the other person to everyone. It tends to fade after 6 months to 2 years.
One of the concepts that I had to take to heart to really understand polyamory is the concept of compersion or frubble. It is that nonsexual joy you feel for your partner's happiness.
I would bet that he does feel just as strongly for you now as he did before. For me, it amplified the feelings I felt for my girlfriend when we had another woman join us. I think I stayed up on a love "high" that transfered from one partner to another. Each one made me appreciate the other one even more.
I would also suggest the standard communication advise. Talk about what you feel to both of them.