I was myself pretty stiff coming out of nearly 20 years of sprint training. Stiffness is considered a quality there :)
When I started doing other stuff, my lack of flexibility really bugged me and I spent quite a lot of time researching the subject. Standard process : stretch for more than 30 seconds, use PNF, use foam roller,..
In the end, I would say two things matter to really make improvements regarding your flexibility :
- Understand that limited ROM is because your body (brain, nervous system, whatever is at play) prevents you to go there to protect you.
Based on 1, you must therefore teach your body 'its ok, it's safe'
To achieve this you must strengthen your body and muscles. Yes, flexibility actually requires to be strong. I spent years thinking my hamstrings were tight whereas they were weak (despite sprinting, due to injuries) as well as my hip flexors. Worked on that, immediately improved my ROM. This might require you to spend sometime finding your imbalances / weaknesses
To teach your body its safe, once you get the physical part covered (strength), you must use tame your nervous system. Use calm, deep breathing. There is ton on the subject, i personally usually recommend kit Laughlin who introduced the concept to me. From there, I learned then much making the connections.