First of all, YOU don't get to decide if she is with him or not. YOU don't even get a vote. YOUR opinion doesn't matter here.
She wants time ....give it, give her more than you have to give. Ultimately you want her to decide she wants YOU! This can only happen on HER terms.
How to hold your ground... by stating you will no longer be in a rel'p as long as she is with another man. This states YOUR boundary.
M:46 H:49 T:20yrs myD:22 H distant summer/12 H sleeping in b'ment: Nov/12 BD: Dec 2/12 asked me begin to move end of Jan/13 moved Jan 7/13 (left my stuff) "agreed" to "working on r" Mar 3/13(lipservice!)