First, I want to start out by saying that when it comes to self-esteem I know precisely how you feel. I too have low self-esteem and it's been a good majority of my life I"ve felt this way but I hit an all time low when I did tip the scales at over 200 pounds. This in no way defines who someone is on the inside and people around you (home or work) shouldn't be making fun of you. Ask for your husbands support and possibly ask him to exercise with you. It sounds like the trainer blew you off. I find that insulting and you should talk to the manager there about this behaviour, but I'm glad you are still going. Keep it up. You will feel better for it and in time the exercise will begin to show. Don't let anyone get you down.
As for your co-worker, again her and the old co-worker's insults were childish and unwarranted. I personally would talk to her but that's just me. If you are too shy or too hurt or ashamed to do so, why not try a different approach. Start a conversation with her (you did say you work closely together) and mention you started up a membership at a gym. Ask if she'd like to work out with you and that a buddy system exercising is one of the most powerful ways to keep someone going and that you'd appreciate her support as a valued co-worker. Maybe then she'll know that you already realize you need to lose weight, is in the process of losing the weight and be more supportive rather than put you down. And this will also keep your working relationship with her from getting to the point you two cannot work with eachother.
To your daydreams of your husbands snuggling, if your self-esteem, etc. is preventing you from doing so, then maybe some sort of counselling would benefit. At least this way you can get to the bottom of why you feel the way you do. Ask your husband to come with you on occasion so he can also gain some insight into the person he loves and is married to.
I do wish you luck in all you do. And I just want to say that I'm proud of you and your willingness to go to the gym despite your self-esteem. You are one of the reasons why the rest of us with low self-esteem keeps trying, because we see others doing so also.