On the one hand, this is a problem. On the other hand, the Russian "frozen conflict" doctrine relies on the fiction that territory cannot be annexed by force (when it obviously can).
One thing which Ukraine could theoretically do is recognise the de facto independence of the rebel areas and once this is done they would be free to join NATO.
Ukraine would thereby yield territory in exchange for stability and a permanent check on Russian expansion.
Hardly an ideal situation, but possibly a better long-term option than the status quo.
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