No, you cannot choose to stop casting a spell once you have announced that you want to cast it. The only way to stop casting a spell after that point is if you are unable to make the choices required by the spell, or if you make a game play error by misjudging your ability to pay its total cost. Either way, Selvala's ability has no relevance to this as it cannot be reversed.
The Comprehensive Rules do not provide a voluntary exit point for the process of casting a spell. Once you did start casting a spell, you follow all instructions as outlined. The only premature exit point in the CR is when a spell becomes illegal to cast, at which point the game is rolled back to the state just before you started casting. This has been discussed in this question
601.2e The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 726, “Handling Illegal Actions”).
This happens after making choices, but before determining and paying costs.
If you made all required choices legally but then determine that you can't pay the total cost (typically because you don't have enough mana available), you have committed a game play error as per the Infraction Procedure Guide, with appropriate steps taken by the judge to remedy the situation and/or give a penalty as deemed appropriate.
That being said, Selvala's ability has no bearing on any of the above questions. Selvala's ability, once resolved, cannot be reversed. It does not matter whether or not you activated it while casting a spell, or whether or not casting that spell got cancelled.
- Handling Illegal Actions
726.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. Each player may also reverse any legal mana abilities that player activated while making the illegal play, unless mana from those abilities or from any triggered mana abilities they caused to trigger was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed.