Hey there MyotherB,
It sounds like you are handling all this with a very level head. That's an awesome approach. It's too bad the first relationship didn't work out as desired, but trust me, "if wishes were horses - beggars would ride" as they say.
One of the things I think helps in general - not only in the realm of relationships, is to not get too excited about potential and possibilities - even when things seem to be flowing in a wonderful direction. Seems for every 'up' there will be a corresponding 'down'. That's not being a pessimist - just a realist. Life is like that. Better to be prepared. Celebrate the 'ups' but be ready for the downs and expect them. If they never come and we die in some blissful state - where's the loss in that ?
Now............on a more practical level...........
It's normal to have ENVY creep into things. You get pleasure from being together as a 3some. For now you may be left out of that. But remember - it's not all about us (you). That's the compersion part you know of. This is something for her. IF it's good - (notice the capitalization) be happy for her. Her happiness will only leak over to you either directly or indirectly. Is it easy ? Hell NO ! But it IS an acquired skill and you WILL get better at it. It's happens rather fast in actuality.
The second thing and again, it's common, is this struggle about whether YOU want another relationship. I advise just making sure you are honest with yourself. You may or may not want or need another. You seem like a kind soul so when considering another relationship you have to consider the possible ramification of that other person too. In other words, that you are going into it for the right reasons and intent.
Third, and this is always a tricky one, is your connection/approval of any of her potential interests. This is fertile ground for control drama. You have to be on guard for that. But on the other hand, as many of us have talked about in other times, there IS a positive in having another set of eyes in the picture. Sometimes we see things of genuine concern that someone caught up in NRE doesn't see. Just make sure that conversations around this ARE genuine. Be careful about not creating monsters where they don't truly exist (because of envy or jealousy). Understand (and explain) that BECAUSE of your love for her - your first concern is for HER ! Concern AND responsibility. If you see wolves on the lawn you have an obligation to protect her. Just make sure those wolves are real ! And you MAY need to bring in yet another set of eyes to verify that
But other than that, you seem like a natural. Just try to keep a level head about all the various things that come up - and they WILL. Go for long walks alone. Keep clarity.
Good luck & keep us posted.