I'm really glad you reached out, thanks for sharing on here. Grieving is so painful and it seems unpredictable at times..
5 months from a breakup is still very recent.. It sounds like agreeing to remain friends and live together has brought about some challenges that are hard, and feelings that you didn't think would come up. I hope you can be patient with yourself by recognising that jealousy is a natural feeling given the history you have with this person.. 5 years is a long time, and especially as you're living with him, naturally you're going to see many parts of his life, as opposed to cases where couples seperate and go about their lives independently. Grieving would be very different as things would be brought back up again, and I don't think either of you are to blame for that at all, moreso the circumstances.
Perhaps the circumstances you're in are making it hard to grieve, and I think it's very normal in a lot of cases for it not to be healthy to know who your ex is now with. You may find that jealousy is very hard to contain and I don't think it's completely fair to have to bear all the responsibility for controlling it.. And I think that's where boundaries become very important when both living together.
I can see that you still love him and want the best for him, and I'm so glad to hear that both your ex and new partner are extremely supportive. Did the feelings of jealousy start when you found out he was dating again, or was it a situation where they were at the house and saw them together?