Generally it's not that important to have a straight leg in a stretch, as long as you feel the stretch. Bend your knee as much as necessary, and as you get more flexible, you will need to bend it less.
It’s OK if you can’t stretch usually this changes from day today and whether your body is warm or not. Start small every day try to achieve the maximum stress you can do without trying to go further. Sticking to doing that for about a month will increase your flexibility. Also if you work out you will usually find that after working out or generally after exercising that Your range of motion is extended and stretching is easier.
I would say go to wherever your comfortable, then if you can push yourself slightly further and eventually you will see progress.
You have to be careful slightly bend your knee to go into a lower stretch I learned this in karate class for 5 yrs that I took it.