I’ve been having this problem for a very very long time.
I got to the point where I had had more than 500 LD’s and only a handfull longer than 5 minutes… and that got to me after a while and I started losing motivation. I think only now I’m starting to get past this obstacle. 
The more detail (and more joy I’ve gotten out of it) I’ve been writing down about my dreams, the better my recall has gotten and I now remember my dreams a lot better and more vividly. I believe this really helps with getting used to the feeling of my dreams. They are starting to really feel like another part of my reality, getting closer to my waking experiences, like two sides of the same coin, instead of two completely different things. This, plus awareness in general, seems to be getting me closer to my dreams, more intimately involved with them and this way they seem to be… stronger and harder to let slip through my fingers. I guess to remember your dreams you need to be aware of them, even if you are not aware in them (if that makes any sense
). Yet awareness is awareness…
Another thing I’ve realized as well is that I’ve been having this problem for such a long time that I got to the point where as soon as I got lucid I’d start worrying I’d wake up. I would naturally expect and fear waking up at any second. Then I’d kinda almost panic and try to do things in a hurry, which just helped me walk up even quicker.
On the other hand, it’s hard to just tell yourself to be cool and expect to stay in the LD for a long time, it’s hard to really convince yourself and completely get rid of the fear. I guess this is one thing that really comes with experience and realizing you CAN stay in a LD longer and you need not fear waking up. Practicing being aware of my surroundings helps here too, as when I focus on the fear of waking up you forget about the dream itself and what grounds you there…
Another thing that would happen is when I did get a longer LD I’d suddenly think “hey, I’ve been lucid for some time now without waking up!” and then I’d wake up right away
So thinking too much about waking up or me being lucid would make things collapse. I’m sure with practice we’ll be able to think whatever we want without waking up, but at this point my lucidity doesn’t seem that stable, in general.
On the other hand, the other night I had a LD in which I noticed something interesting (and probably hard to put into words). As a DC was talking non-stop I got bored and realized that my lucidity felt like it was “in the back of my mind”. In my LD’s I’d usually get lucid and hold onto the lucidity desperately and, as I said above, focusing too much on the fact that I was dreaming seemed to make things fall apart. But in this LD it was like I got lucid and put that information in a safe place in my head where I wouldn’t think too much about it, but still be aware of it’s presence. It’s hard to explain 
Maybe LL helps too (I mean, I bet it does). Maybe something like… IWL I look at a tree. In my LD’s I would use to look at the tree and think “I am dreaming”. Maybe the “LL approach” would be to get the “looking at tree” and “thinking ‘I’m dreaming’” and put them together into “looking at a dream tree”. Does that make sense? Maybe every time I thought about my lucidity I’d lose focus on my surroundings and that would help make things fade. In this new way of looking at the tree the lucidity would be implicit and not require any actual thinking about it.
sorry for the wall of text, I got carried away.
if this doesn’t help, no problem, it did help me! I realized some things that might help me while I was typing lol!