I don't have a ton of experience in this area but I'd say just keep doing what you're doing! I do the same as you - 1-2 sessions of intervals a week on the trainer (I'm afraid to do sprints near cars), plus once a week I'll do a speed ride where I go a shorter distance than usual, but push myself just a little harder than I normally would. I've been seeing a gradual increase in speed with this method. When I got my road bike in mid-August I was averaging between 12-13mph, and now I'm closing in on 16! So stick with it! Keep your confidence up and don't give up! And don't feel bad about big guys passing you. Just tell yourself that you'll be the one passing them eventually. I actually had a pretty embarassing moment the other day - I passed a big strong looking man on a long climb. I was so proud of myself, then he just whizzed by me on the descent! I was kind of expecting it (I think the big guys have more momentum and can descend much faster) so I just had to laugh at myself.

Also I'd say yes to the fitting. It's totally worth it. It will maximize your efficiency as well as overall comfort.

Happy training