I was ten years into Vipassana before trying the Jhanas and found them challenging. two years practising, I eventually got them. I found leigh brasingtons stuff very useful on it, though I think he was born with the "concentration" ability somewhat, while others like me have to struggle harder to get there. https://www.leighb.com/sitemap.htm

he also had a clinical study done while he went into the jhana states that is worth a look. that can be found here - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3659471/