Firstly, it's perfectly natural to feel this way about someone who is showing you kindness and helping you to achieve your goals. As humans we're wired to seek people who improve and comfort us and that's a personal trainer's job. Secondly, don't be so down on yourself. You're doing something great for your health and well being which shows a great deal of determination and strength of character. Fat does not mean ugly and plenty of disabled people are in happy, committed relationships.
In saying that, though, I do think nothing will come of a crush on a personal trainer. Part of their job is to be flirtatious and overly friendly to keep people coming back and paying for sessions. Many of them are actually married but take their rings off for work. My partner used to be a bartender and his managers told them to appear single and available to reel in more customers. So it's quite likely that people in jobs where attractiveness pays will actually be unavailable, either due to marital status, sexual orientation or outright not desiring a relationship in general.
