Oil, the fatal weakness
in the Summer of 1944, the USAAF concentrated on German oil production and reduced it by 90% by the fall of the 1944. This was a permanent loss, as every time the Germans tried to rebuild it Allied bombers hit it again.
US strategic bombing survey
http://www.anesi.com/ussbs02.htm#taoo
This in OTL had catastrophic effects for the Germans.. German flight operations were severely limited and then reduced to only the most urgent, while major offensives (like the Battle of the Bulge) failed in part as there was simply not enough fuel to maneuver.
Even assuming the Germans have restored captured oil fields in Baku, Grozny etc, they still have severe problems. Distances are huge from Germany to that area, the infrastructure was designed to send oil to central Russia not to Europe so has to be built from scratch, and ultimately it isn't the production fields that matter, but the refineries, and those are relatively vulnerable targets to World War II era bombing raids.
Plus of course the Allies can hit Baku etc just as easily as they can any other German target.
The earliest that the Soviet Union could collapse would be the winter of 1941-42 assuming Operation Typhoon takes Moscow. Even then it would take at least another year simply to mop up and establish a firm line of control, and probably longer. But more likely a Soviet collapse would be sometime in winter of 1942-43 based on the assumption that Case Blue is a spectacular success and Stalingrad falls, the Soviet counteroffensive is destroyed, and the Germans secure the Caucasus region. (there are plenty of alternate history works that talk about how that could happen, some better than others)
So again, figure a year to mop up or more, and thus no significant freeing up of German resources until 1944 or later. Meanwhile the War against Japan continues, and the Allies are still going to be securing North Africa. Without the Soviets, their most likely next move would be to start a campaign in Southwest Asia to make sure their oil (Persia) is not taken out, and to allow some supplies and help to reach whatever post Stalin government survives in Soviet Central Asia and Russia east of the Urals. In OTL considerable assets were sent their just to build a supply line, and if instead of an Italian campaign this is done, it well within Allied capabilities while being a long way from the Fatherland for the Germans.
The Allies might just wait on Germany at this point, and focus on taking Japan out sooner but it wouldn't strictly speaking be necessary. Plenty of resources already available for that. So with forces already committed to dealing with Germany in OTL, the Germans still face the problem of a running sore in the East, while still having to face the possibility of invasion from the West, South or even in Norway. While their fuel supplies are hammered as well as their air force and transportation links.
As is, 2 million German military personnel, plus tens of thousands of guns and most of their fighters were facing the Allies in OTL by 1944. They don't have that much more to commit, even without a Soviet Air Force and Red Army to deal with.
The Western Allies are still very much in the fight even without the Soviets
it just is a lot harder.. especially for the Europeans