Hi all, looking for some advice.

Current situation: My lo is 3.5mo. Been co-sleeping since baby was born. He didn't like his bassinet and co-sleeping was the only way we would get sleep. I nurse him to sleep. When I put him down after he's asleep he usually wakes up, so I give him his pacifier and will fuss but will usually fall asleep with his pacifier and spit it out shortly after. He'll usually wake up again so I'll have to either nurse again or give him his pacifier. Then he's usually sleeping for several hours. He also hates car rides and cries to the point of being hysterical and then will sometimes end up falling asleep in the car after crying so long (15-30mins). I feel bad about it but there's nothing I can do when I'm driving. I'm assuming this will also be his reaction to sleep training.
Ideally, my goal is for him to fall asleep on his own. When should I start sleep training? Which method may work best in this situation? How long did it take you? I'm not set on one particular method yet, but I'm feeling guilty already about him crying and feeling abandoned. I know he has to learn to self soothe though