You sound really frustrated with yourself and I don't think that there is any need. It seem to me like it is perfectly reasonable to be bothered when somebody close is starting up a new and potentially important relationship. These things take time to build and as there is only so much time in a day, spending time building something with a new person might (might not, of course) mean less time for the more established relationship.
It's how you deal with it that matters I think. You always seem to be relaxed, happy and committed to dealing with any problems in a constructive way. So I think you will ultimately be fine.
I just hate to hear of you beating yourself up for not being fine right now.
I hope that life seems more normal for you soon.