Hi Bunks, and welcome to Dhamma Wheel.
I wish I could say otherwise, but there is no quick fix here. In my experience, it is going to take time to overcome lust as a hindrance. So right away I advise you to avoid self-recrimination and guilt-tripping yourself
over it. If lust arises in the mind, don't blame the mind - it's simply repeating it's conditioning, the conditioning it's had over not only this lifetime, but (if the suttas are to be believed) countless previous lifetimes also. Every time in daily life that we look lustfully at a pretty woman, indulge fantasies, pleasure ourselves, or engage in sex, right there
we are conditioning our minds in that direction. No wonder
it springs up in meditation - we are the ones who put it there in the first place!
So we need to look at what we do in our day to day life. If we cultivate lust, it will spring up in meditation. If we work on alleviating it, trying to gradually cool our tendency to indulge lust in daily life, it will, over time, spring up less and less
in meditation. Sitting around feeling guilty and ashamed won't help us. Doing something about it,
in whatever way we can, will.
But in the short term, there are two things I've found useful. One
is to immediately 'knock out' the lustful thought with a more skillful one (for example, bringing to mind the inner body parts of that object of lust. Be as graphic as you need to be, they are just images conjured in the mind so they can't hurt you). Once the lust has subsided, go back to the breath. The other is,
to cultivate the ability to breathe in, and out, aware of the entire body at the same time. If your mind is on (or in, or permeating) your body as it breathes in, and as it breathes out, you can begin to cultivate a calm, soothing pleasure that, while more gentle and refined, gives your mind something non-
sensual to enjoy. You can gradually begin to take pleasure in meditation, which will lessen the hold that lust has on the mind. But once again, progress is gradual and patience is needed.
Here is a Dhamma talk that I have found very helpful: