I have to say that while I agree with most of the above posts that filling your free time and keeping busy is a healthy way to respond in general, I also recognize that the reality of my situation is that I'd be really hurt if I were in your shoes. It may well be rooted in my own insecurities or fear of abandonment, but I have to acknowledge that if my boyfriend started giving away our normally scheduled time to a knew love I'd be upset. Especially if he didn't check in with me about it first.
My boyfriend and I have one night a week that is scheduled as our time and we both value that time. Sometimes we'll mutually agree to include others (the other half of our quad or other friends) but it's definitely something we talk about first. Occasionally special circumstances arise and schedules change. I know that soon we will miss our night so that he can go see a new girlfriend. But it was something we talked about and I was supportive of. Totally different in my opinion. I say this not to add fuel to your fire, but just to let you know I understand how you feel. It may be that you just need to wrestle with your own insecurities and figure out what feelings the changes are bringing up.
Now, as far as the new gf not knowing about you, that sets off red flags for me and changes the ballgame completely. This is not just about insecurities but ethics and trust. Sounds like it's time to have a talk. . .