I'm not sure if this will help, but this example I will share, while light (and by no means the only example I have, as you know based on previous posts of mine that I struggle with the same thing), is something I've learned recently.
I am moving soon. I think that's well known by now, considering how much I've posted about it.
Equally as well known is the fact that I liked/was-trying-not-to-like this guy I had met here. I've had the biggest crush on him for a year. I read into things, and maybe subconsciously pushed other things away, but I'm realizing with this move, that I wasted all that worry, all those emotions and thoughts, for
nothing. The flesh of my heart wants to say, "Seeee? THIS is what happens, NOTHING, this is why you don't go down this path. NEVER like another guy EVER again, Rachel." Which obviously I am terrible at that, I get crushes way too easily.
That may sound like guarding my heart, but it's not. That's trying to desensitize my heart by telling it lies. That I'm not worth it, that I'm not pretty enough, thin enough, whatever. That's putting up walls by tearing myself down. Or, tearing others down while putting walls up: "Everyone is like that, no one is truly kind, all women/men are terrible", etc.
Guarding my heart would be taking everything to Christ (which I usually don't when I get a crush, I just sort of run with it), keeping sin at bay while letting lessons and love in. Guarding against flattery, lust, bitterness, while letting in loveliness, peace, and joy. It's...hard. Definitely.
When it comes to particular people and not just in general, I think it takes time and observance, true observance not blinded by emotion, which is what happens at the beginning of a lot of relationships, be it romantic or not.
I don't know if that was helpful, just what's on my mind currently.
