What do you suggest then?
Give up hope that this war will end in a clean or timely fashion. Accept that after the dust has settled the country will be all but destroyed.
Deny Russia any profit from its intervention. Have our proxies shoot down Russian planes, and make sure every single Russian mother understands that the coffins coming back home are the direct result of Russia's adventurism. Make sure not even RT can put a positive spin on what is happening.
Bleed Iranian and Hezbollah forces in a similar way. Make sure the pictures of dead Iranian generals get decent exposure.
Communicate clearly that Assad needs to go, that there can be no peace with him still in power.
Accept that these goals can only be reached by creating further destruction. Accept that this destruction is sad, but necessary for a lasting peace.
Communicate clearly to Russia that we are inclined to respect its interests in the region if it joins us in creating a solution that is acceptable to all sides. Make it clear that we do not intend to firmly embed post-war Syria in our sphere of influence.
Communicate clearly the same thing to Iran. In particular affirm that it is one of our core interests to see the Shia population of Syria protected from Sunni revenge. Offer Iran a big slice of the cake that is political influence in post-war Syria.
Communicate clearly that it is not our goal to eliminate the entire political machinery of the current regime. Our objections should only be directed against Assad and his inner circle - they can serve as a scapegoat for others' crimes, at least to a degree.
Insist that any post-war order in Syria must include a
Truth and Reconciliation Comission like they had in South Africa after the end of Apartheid. For the peaceful coexistence of the various ethnic and religious groups it is vital to break the circle of revenge and violence.
Abandon the idea that Syria must survive as a nation-state. Start looking for a solution that gives the various groups a great deal of autonomy, with the central power by design unable to oppress any single one of them: Nobody will have a reason to revive inter-group tensions if all the power they can want already is distributed among their own group.