So, the Nordic hamstring curls. I also was not strong enough to do them well , however you can fall and catch yourself with your arms and hands and bump yourself back up. I think there is a video on how to do this. It's pretty awkward though , that's why I rigged up the inner tube and the roman chair works pretty good too. I got really strong at that.
That's the one at the gym that holds your heels and you fly out like superman and hinge down and pull back up. Really good exercise.
With the inner tube: It is hanging from the shelf behind me and I reach up and grab it with my hands and hook it over the bottom of my foot . The other leg stays laying on the floor straight. With foot plantar flexed I push the band taught by straightening the leg I am working the band with , the leg will be pretty high off the floor and then I pull my foot with the heel leading towards the ground the band gets really tight and then you slowly let your leg get back up to where it started and repeat for reps. This not only helped me with the trouble you are having, but I hit old scar tissue in the big part of the hamstrings with it whenever it starts to bother me.
I hope that make sense.