All this is perfectly normal. It happens to everyone when learning to fly gliders.
Went down to the club to fly and was nervous all day before I flew ... The only thing I could put it down to was that I knew I would be flying with an instructor I had not flown with before.
That will disappear once you've flown with a few different instructors.
A tip - you will find that different instructors work better for you at different stages of your training. Unless yours is a big club you probably can't choose, being stuck with whoever is on duty that day, but it helps to recognise this. If there are two or more instructing, you could always ask to fly with X today (something like "I find X's explanation of [whatever you're working on] easiest to understand" would be tactful). But don't not fly because only Y is instructing - ask to work on something different, and you might find that Y is the best for that (for you).
I don't know what went wrong but my co-ordination was completely out, and then when we were on approach and about to land I completely failed to round out in time and had to be rescued by the CFI on the controls.
Very common. You did landings last week, now you're working on (say) stalls and, guess what, you've forgotten how to land. Brain overload. Keep at it. It suddenly comes together and you can't imagine why you ever had a problem.
I made a better approach next time, but still missed the round out. The ground seemed to come up very fast and again the CFI took the controls to ensure we didnt hit the ground hard.
I went through a phase of this - my instructors must have had nerves of steel to let me continue and land the thing. The problem is usually that you are looking at the point on the runway at which the glider is aiming. The trick is to switch your attention to the far end of the runway (or at least half way along it) at around 50ft, and start very gently easing back on the stick about then. If you're looking at the runway in front of the nose you can't judge height, and suddenly the ground rushes you. If you're looking into the distance you can see the aircraft settling and ease back on the stick as needed. I suggest you ask your instructor about this (explain the problem next time before you fly), and maybe ask for a prompt to move your gaze at the right time.
Last week my landings were good, in similar conditions, similar wind (although I was in the Grob Twin last time, this time I was in the K13).
Gliders are supersensitive to tiny changes in conditions. As an example, a 10 degree change in wind direction might put you in curlover from a hedge or some trees, so that you're flying in sinking air for the last 25 ft - that makes the ground arrive pretty rapidly. Also, a K13 flies nothing like a Grob Twin - if you were doing the same things (as of course you would be at 1.40 hrs), it's no wonder it didn't behave the same way.
Overall, glider flying is about making constant tiny adjustments to keep things looking "about right" (a phrase you will already have heard). It really is just a matter of experience - eventually things like when to round out become entrenched and don't require deep concentration, and that allows you more brain space to think about other aspects of your flying.
If it helps, I started gliding aged 40 and went solo after about 9 hrs (and lots of launches and landings) - about average for my age. It wasn't until about 6 hrs that I had real control of the aircraft, and then it just all seemed to come together at once.