I am probably the worst person to give you advice about this situation. I bought Soda before I could really afford him.... and honestly its scary & stressful to know that if something happens to him I will have to depend on my parents or somebody else to pay for anything major. I'm finishing up college, living at my parent's house, and right now I make around $10K a year. Frankly its stupid that I own a horse, esp in this economy where I have no idea what is going to happen after I graduate.
That being said.... I love finally having my own horse. I haven't been able to ride him for the last couple of weeks, but just waking up in the morning and seeing him out in the pasture and hearing him nicker when he sees me brightens up my entire day.
But, I'm very lucky, my parents have land to keep him on, so no boarding expenses for me. And my dad really likes Soda. In fact, he got on him this last Xmas and took him for a ride. The first time he has been on a horse in 10+ years. As he told me right before I brought him home "Nikki, you're my daughter and you know that if you need help with him, we'll be there for you. So if you need to bring him home, we'll figure it out." Long story short (sorry I ramble sometimes) if I didn't have my family behind me on this there is no way I could manage it. I'm very lucky that they came around too as when I finally told them I bought a horse, I almost got kicked out of the house (I had owned him for about 6 mnths at this point). And honestly if I knew the position I would be in right now with Soda, I don't know if I would've bought him, no matter how much I love that horse. But, now he's mine and in my family (like yours) once you buy an animal you are responsible for it for life. Not just for as long as it serves your purpose.
I guess you have to figure out if your trainer can be counted on to help you if something happens to her.... or if you fall on hard times. Sorry this probably wasn't very helpful.....
I feel for you though, its a hard decision to make.