Don't be discouraged by your weight gain, make sure you get bloodwork done to make sure you aren't fighting another kind of battle.
I'm going to purchase P90 to get my butt in gear, but the suggestion to eat as if you have Type 2 is an excellent idea.
Are you an emotional eater? If so, try and limit what you can grab at home. Stock your fridge with the good kinds of snacks and throw out (or give away) the sweets. So much easier said than done, but remember how you feel after you eat junk food. (It won't always stop you, but it helps)
Allow yourself a cheat day, don't blow your entire diet that day... But just eat a piece of junk food or two.
Good luck with your search for a new horse. The fact you are concerned that you're too big for him will weigh on your mind whenever you ride him (even if you were fine) and will turn riding into a chore instead of enjoyment.
Fiona takes care of me and I'm a careful rider, but I'm still incredibly self conscious to the point it'll sap any fun I'm having if she acts a tiny bit off. Saddle fits, but I am upwards of 200lbs and last thing I want to do is hurt her with a careless jump.
Keep your head up, good luck!!!
Willow - 5 y/o Gypsy/Haflinger Mare
Loki - Yearling Gypsy/Haflinger Colt
Rockstar - Senior Spotted Saddle Horse