Let's not hijack the thread. Right now we are talking about ruxpinsam's dilemna .
First of all, with regard to your boyfriend's comment about losing money:
The amount of time and bother it will cost you to sell the gelding is something to consider. If you have a ready buyer, even if they are not paying top dollar, you should weigh the convenience of this against the possible gain of trying harder/longer to sell on the open market. Time IS money.
Secondly, do not let the size of the mare scare you. Some 17 hh horses are just so wonderful to ride. They can be very gentle and very solid feeling. It's actually harder to stay over a squierrely 14 hh horse than over the same squierrely 17 hh. I mean, you have more wiggle room before you fall off.
On the other hand, once a big horse's engine kicks in, it can be a lot to deal iwht. I only wanted to say that just becuase a horse is big does not mean they are more challenging. I am riding a 17 hh horse and other than the challenge of mounting, I ADORE the big lug.
Leasing a horse , if the owner is someone you click with, can be very reassuring , especially if you want a little morale support, or if you have questions. Would the owner be available to you and could she give you some lessons?
The way you "click' with the horse is much more important than the size. And the human relationship , with the owner, if good, can be a wonderful help to a person who struggles with confidence.