I would definitely agree with trying to relax - I have had this problem myself! The more tense you get the less balanced and forward thinking the horse will be. It sounds like this horse takes advantage of that to run on instead of changing pace.
Would also agree with a lunge lesson or perhaps...
I can highly recommend this site for info about pony books and buying them second hand:
http://www.janebadgerbooks.co.uk/
I'm not connected to her honest!
The guidelines say that there is no particular reason to stop, except that your centre of gravity changes from about 5/6 months gone and you may feel less secure in the saddle.
i am 13 weeks and still riding 3 times a week. I have given up jumping and hacking alone, but still do schooling and...
Ziggy has some really good ideas there. I would also say, if she is specifically becoming attached to the YO's horse, could they be put in different fields? I would try and avoid pair separation anxiety if possible.
that's good news - well done! I have found in the past few weeks that my horse (who avoids right canter) is much more reliable when asked from a really strong bouncy trot - maybe that is similar to your situation (e.g. he is more forward when jumping)