Don't be. This merger being blocked means JetBlue now won't acquire Spirit at an inflated valuation. Instead when the company is liquidated they can buy the airport slots they so direly need without all the extra debt -and- they get to pick through anything else they want like airframes. So your favourite airline will likely get everything it wants (seems like they were willing to pay more than Frontier anyway) with nothing it doesn't. The only downside is they have to wait for bankruptcy proceedings which are about 4-6 quarters away looking at Spirits run-rate and revenue projections.

However you should be sad in general because this is a terrible judgement that dooms a business from taking the only way out that would have been a reasonable outcome for its employees, shareholders and users.