There isn't any one specific feel-good chemical. Serotonin, dopamine, acetylcholine, & norepinephrine all play a role in emotions, as do endorphins. There are no supplies of "happy chemicals" that we could rely on to fix our emotional issues, unfortunately.
I would think that the blockade of dopamine D2 receptors plays a role in why Invega victims feel like their recreational activities are pointless. Dopamine release has been recently proven to be a factor in how we decide what's worth expending effort on (or not). Lack of normal DA activity leads your brain to decide "[activity X] is pointless, why even bother, you're not going to get anything worthwhile outt of the experience" - even where "activity X" is something you previously considered fun.
One study showed that amphetamine will reverse the antidopaminergic effects of haloperidol, and restore motivation in rats:
an injection of haloperidol 48 h before treatment, followed by saline 10 min before test, significantly reduced preference for high reward/high-effort arm, giving the same haloperidol injection followed by amphetamine 10 min before testing blunted the effect of haloperidol, and completely recovered preference for high reward/high-effort arm (Bardgett et al., 2009). Evidence therefore points to amphetamine's ability to overcome the effects of DA blockade induced by haloperidol. (ref)
I'd bet that'd work for Invega too. That said, giving someone on an antipsychotic regular doses of amphetamine could end rather poorly, especially if the Invega is ctually being used to treat schizophrenia or other delusions/behavioral problems. It's not something I would recommend every Invega user consider: ideally you'd use amphetamine as a stop-gap to get your lifestyle back on track, to get through the extra gray days. Amphetamine use won't be a silver bullet miracle cure - it won't produce permanent changes in functioning - but if used responsibly could help you get through the 6 months of waiting.
Other dopaminergic stimulants might also work. 3-fluorophenmetrazine, methylphenidate, 2-FA, maybe even some of these cathinone RCs. I'd avoid cocaine and methamphetamine for obvious reasons. Another idea maybe would be to get on Wellbutrin (buproprion) - it's a NDRI drug and can help boost the amount of norepinephrine/dopamine in the synapses. Even caffeine may be worth a try.
Just be careful, and be mindful. Taking a potent stimulant to counteract a potent antipsychotic could end negatively if you're reckless wih dosing. You just might end up with a stimulant dependency afterwards! Stick to the milder compounds (caffeine before methylphenidate, methylphenidate before amphetamine, Dexedrine before Adderall) and keep doses low. Only dose when you really need a boost. You know the drill.
Aerobic exercise, a healhy diet, plenty of novelty. And time, lots of time.