Hi Lydia

Welcome to Social Slimmers
I completely understand where you're coming from about the motivation, I've been on Slimming World for several years, and once got to target, but put it all back on again. It took me a while to actually get my motivation back again, but I've found myself back on plan recently and I hadn't realised it properly. I think what gave me a kick up the backside was that I'd gone back to (and slightly over) my original start weight when I joined SW. I was so disappointed with myself because I allowed my realtionship with food to start taking over again, when I'd had pretty good success in changing it and feeling so much better about myself.
If you don't want to join a particular diet/healthy eating plan, why don't you focus on portion control? For example, on SW they allow you to eat what you want of vegetables, gruit, meat, fish, eggs, fat free cottage cheese & natural yoghurt, and also pasta, rice and potatoes, but the downside with eating load sof pasta, rice and potatoes is that they are really stodgy, so a lot of people monitor how much they eat. I make sure I never have more than the recommened portion amount (usually 75g of pasta) and try to fill my plate up with veggies and meat.
There are plenty of food diaries and journals on here for you to have a nosy and and pick up some ideas if you decide to go towards a particular plan.
Like Matt said, 2 stone in 9 weeks may be too big of a target, but say you lost 1-2lb a week, thats at least half a stone, possibly 1 stone before you head back to Uni. If you aim to lose too much too soon, you could end up becoming disheartened if it doesn't happen, and that can just send you off track completely and make you more unhappy than you may already be.
What about setting small goals, like losing 10lbs by the middle or end of August? That way, if you lose more, it's a bonus, but you've still got almost half way to your target weight?
Hopefully we've been of some help to you, and I hope you get your motivation back and get to where you want to be

Good luck! x