TL, DR, the war may be swinging in Russia's favour, and time may be on its side for a variety of reasons.
I feel like the problem is being overstated.
There are some countries where I think it will matter. I don't think the UK government is going to maintain the same level of support once people stop being interested. The invasion was a convenient political tool to foster the kind of jingoism the Tories get off on, but ultimately Boris came into power partly with the aid of Russian disinformation campaigns, and I don't think his government have any principled stake in this conflict at all.
But frankly, the UK contribution so far has been kind of pathetic anyway. I don't think Western European countries were ever going to send much more than they already have. The UK does have a lot of good quality military equipment, but it doesn't have huge stockpiles of equipment that isn't being used, and if it comes down to a choice between Ukraine losing or the army being noticeably weakened, I don't think any degree of support for Ukraine would make that choice politically viable.
In terms of who actually is doing a lot. Obviously there's the US, but the US has vast stockpiles of cold war era military equipment that it was just going to decommission eventually anyway. For all the talk of the stinger and javelin missile reserves being depleted, the US military doesn't really have any need for them. There's never going to be a situation where the US fights a war without massive air superiority. The US basically loses nothing by sending its military surplus to Ukraine, so it's kind of a no brainer to do so. At absolute worst, it might mean certain systems have to be replaced quicker, but the military industrial complex isn't going to complain about that.
Then there's Poland, which has made very clear they are ride or die for Ukraine (probably because Russian state TV keeps putting them next on the list for "denazification"). Poland have already put far more on the line in terms of supporting Ukraine than basically anyone else, certainly relative to the size of the Polish economy, and I don't see that changing because frankly, if it was going to, it would have already.
It's the same story in the Baltic states, they just don't have the same resources.
So yeah, support dropping off from Western Europe is likely to be a problem, but again these countries weren't doing much anyway and have probably given most of what they were ever going to give. I can't see support dropping off where it actually matters. At the same time, Russia doesn't have infinite stockpiles itself, and the stockpiles it does have are in pretty shitty condition due to the ridiculous levels of military corruption.
I'm not saying everything is fine, or that Russia won't ultimately win the war of attrition. Frankly, if NATO wants Ukraine to actually win it needs to do more, not sustain the same level of support. But I don't think we're going to see Ukrainian army collapse from lack of support or equipment.