“If Ukraine can bleed Russia and the rebels enough over the summer they can stall the offensive and possibly make some gains.” Looks like from Erick Falkenhayn who believed “to wear down the defences of the French and bleed their army white”. With the success we know. The question is why the Ukrainian would not bleed as well, as the German did in France?

“Russia, as the aggressor, is fighting for gain whilst Ukraine is fighting for it's survival.” The problem is you are still thinking as Russia is interested to take Ukraine when all evidences suggest otherwise, at least for the moment.
Russia is “just” keeping the rebels the nose out of the water, balancing the forces, so material, political support, probably some cash, volunteers, etc. Ukraine can’t win, so NATO can’t come in, as to join NATO, you have to be at peace. You want peace in Ukraine? International treaty guarantying Ukraine will never join NATO.
Remember, you are one who see Russia as an enemy (from the Cold War time) so there are no reasons why Russia couldn’t see NATO as an enemy as well.
And, actually, Russia has more circumstantial evidences for that than you have of warlike NATO actions to be confirmed in its suspicions.