I agree with what Gustavo says, and I'd like to add a few thoughts of my own.
1a: I wish I could lead a healthier lifestyle.
1b: I wish I led a healthier lifestyle.
As Gustavo says, (1a) is used by someone who wants to lead a healthier lifestyle, but can't; something is preventing him. The same person could say (1b) instead, but there are a number of other circumstances where (1b) might be used. For example, he may have just noticed how bad his habits are, and feel ashamed.
Kes, you wrote:
The small difference here is that there is some problem which doesn't allow the person to be healthy.
This is a subtlety, but neither sentence addresses whether the person is healthy or not. He may be in absolutely perfect health, but still wish he led, or could lead, a healthier lifestyle for some other reason. Maybe he thinks it's just the right thing to do. Maybe he realizes that he's been extremely lucky in terms of health and wants to set a better example for his son, who probably will not be quite so lucky if he follows in his father's footsteps.
But yes, you are on the right track.
For what it's worth, regarding your typo, "lead" can be confusing for native speakers and ESL learners alike. (I have no idea which you are. Most of our members are non-native speakers, but your writing suggests that you've been speaking English for a long time.
"Lead", which rhymes with "seed", is the present tense and simple infinitive of a verb whose past tense and participle is "led", which rhymes with "bed". But "lead" is also a noun referring to a heavy metallic element, and is pronounced exactly the same as "led". This has been the cause of many spelling errors.
In 1968, guitarist Jimmy Page of The Yardbirds was trying to put together a new band to fulfill contractual obligations. He assembled an ad hoc ensemble consisting of drummer John Bonham, bassist John Paul Jones, and vocalist Robert Plant. The story goes that Mr Page's friend Keith Moon (drummer for The Who) snidely predicted that this band was doomed to failure, that, in fact, they would "go over like a lead Zeppelin".
Mr Page and his associates, who had thus far been performing as "The New Yardbirds", decided that "Lead Zeppelin" would be a great name for a rock band. However, they made the decision to spell it "Led Zeppelin" to make sure that it would be pronounced correctly. They didn't want people to pronounce it in such a way as to suggest that they were leading other Zeppelins. It apparently never occurred to them that changing the spelling to "Led" not only erased the possibility that they were the leading Zeppelin, but established the fact that they were the Zeppelin that was being led by others.