When I need some motivation I like to watch motivational videos on youtube. They get me pumped up. You should watch them before your workout or whenever you are feeling you need a kick in the tootsie.
Here are some of my favourites: (Some even have horsies!)
Another thing I like to do is make specific goals. Not necessarily 'weight' related but performance related. Which I think would be good for you since you are still young and its healthier to be more concerned about your health then a certain number.
Some examples are:
- do 5 minutes straight of 2-point at the trot
- do a whole warm-up no-stirrups
- walk route x which is very hard and hilly without getting tired
- try snowshoeing (not necessarily performance related but its great to try new things and they can be quite challenging physically :) )
Another thing that helps me is making lists! Theres nothing better than crossing off something on a list for having completed it.
Get a work out partner, when you are feeling demotivated they can drag you along and vice versa.
Take progress pictures every month from the front, side and back. When you are lacking motivation look at where you started and how far you have come.
Don't use a scale! If you want to measure your weightloss get a soft measuring tape and keep a biweekly-monthly record of your bust, waist, hips, thighs, calves, neck, biceps, wrist, etc. What you will probably find (especially at your age0 is if you start working out, your body will make muscle at the same rate as you are losing fat. Since muscle weighs more than fat it appears you aren't losing any weight but really you are losing fat and gaining muscle. A lot of people get discourage when they look at the scale and it hasn't moved. So thats why I say don't bother using one.
Join a sports team if you can! Exercise will help you lose weight and get healthy. Its a good way to have fun while doing it too :) I wish I had joined a team of some sort.
I know you didn't ask for advice but one thing you want to make sure that you do is cut down on any pop or juice you might drink. Its okay once in a while but you want to be mainly drinking water. Staying hydrated is very important.