With regard to being a "second choice" -- I went through that too and talked about it a lot with my MC. Here's where I came out -- even if you are a "second choice" in the moment that he comes back, you're still getting a chance to participate in something better with him. That's your starting line for becoming a first choice, or someone only a fool would leave. If you can accomplish that, then a year down the line "why" he came back, or in what context, won't matter, because you'll be confident in your relationship. It does hurt initially, and there is a lot of self-doubt that comes with it. You need to deal with that yourself because if you feel like a second choice you will be one. If you can be your best and feel good about yourself, you'll know you're worthy of being a first choice.
WRT the mortgage payment, if you can cover the mortgage yourself, then plan to do so. If he continues to contribute then great, but if he doesn't, that's what you expected and planned for.
If you can't cover it yourself, or don't think that would be fair, then I would write up a list of all the logistics you want to discuss with him -- mortgage, property taxes, everything else. Make it your "for now" list, not a divorce list, and then review it all at once. That conversation may not be fun for either of you so do your best to do it once versus having to come back to it.
Accuray
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015