Yikes. That's no fun. Sounds like love. My thoroughbred gelding has a similar obsession with one of my mares. It all goes back to the basic order of a herd, and who is boss.In my experience your filly sees that gelding as her "man" he's the boss in her eyes, he controls where she's going to put her feet, and how she is to act. When she is taken away from that, she feels as though her leader has been taken away and she has trouble being the "leader" This is what happens to my gelding, he is a follower. In that case, you need to show your filly that actually, as long as you are around, YOU are "the man!" Of course, that isn't as easy as it sounds. Those leading excersises sound good, anything that can get her mind focusing on a job, and you leading is helpful. My gelding and I went through a period where I got a little scared to handle him. Sounds like where you are at. They get a little disrespectful, and the fact is, horses are bigger. What really made the difference in my gelding was this:
I put him in a round pen, in a indoor arena where he couldnt see his mare, and it was hard to hear her. So for about the first 15min, he raced around, pacing at the gate, going crazy whinnying, kicking, all of that. This is a prime example of a horse being in need of leadership..before he got to the point where he could endanger himself, I stepped into the ring, and he continued this behaviour, no respect for me. My favourite training tool is a torn plastic bag on the end of a dressage whip. I use this tool as a question. It does not always mean move, and is not to scare the horse or anything. I began slowly shaking it, point downwards at my side, and increased the noise until he began to tune in. After a while, he actually stopped, and looked at me. So I stopped shaking it. He return to his crazyness. I repeated the shaking. It only took about 4 trys before he was very aware the shaking bag was "asking" him to focus on me. He then tried the old "join up" trick of coming into the middle for a pet. I gave it to him, but shortly after he began pacing and focused on what was outside of the arena. So, shaking bag, he stops and looks. This time, I gave him verbl praise, and asked him, with the bag, to back up. I did this by making my intention very clear in my head, and through my core. I used the bag, very slowly, while moving towards him, with lots of forward motion, to encourage him to back. We continued with these sorts of things, and eventually I moved him around the pen. As soon as his mind went away from me, I brought it back, either by one of the excersises I already discribed, or moving him around the pen, getting him to change direction, stop and back up, using my intention. I focused on this mainly in the farthest side of the pen, away from the gate and door as much as possible. After about an hour of this, I would do "leading games" around the arena, using my little tool, until he had forgotten about his mare, and I was the one leading. We spent about a month doing this consistently, but he improved amazingly even with in a few days. It has gotten to the point now, where he is excellent without her, in fact, at his best. When he is with her, for example if both of them are being ridden together, he will tend to forget that he is to choose me as the leader. Then, I bring back basic excersises to remind him gently.
I know this is long, but I hope it helps. If you have any questions about this, just message me.