I'm going to chop up your post a bit to put parts together, please correct me if this ends up putting things out of context.
Originally Posted by jerflan
...Recently though, we have been going through a rough patch emotionally. He feels I am not supportive, and I feel he is disconnected. We talk a lot but seem to go in circles and have arrived at this awkward and uncomfortable stasis....
... I was very threatened, expressed this, and was told not to worry. I pushed, and got the same result. ... It sparked a huge argument/discussion where I found my fears weren't completely irrational. ...
... I want to trust my boyfriend, but I can't ignore what my instincts are telling me.
I clipped these to leave the "other guy" out of the equation and point out some things that you are saying about your relationship itself.
You two are having an emotional rough spot. (Never a good time to bring others into the mix).
You felt threatened, were told not to worry, then found that your fears had a basis. (Your instincts were correct and now trust is broken).
Originally Posted by jerflan
He says he wants me to "fight for him." I don't know what he means for that. I thought 3 years together meant I didn't have to compete for his affection ... If we have problems, I'm all for hashing them out, but this is too much for me.
He says he wants you to "fight for him" - on the surface this sounds like a manipulation technique to get your attention. Like the teenage girlfriend that flirts in bars to make her teenage boyfriend jealous to make him "prove" that he loves her. Unfortunately this is a relationship dynamic that we see modeled all to often - in life, movies, tv, popular song - it's inherent in our culture.
What he may be trying to say, but not have the skills for, is that he wants you to "romance" him again - to do the things you used to do when you were first interested in each other. He may feel taken for granted and unappreciated. Your statement "I thought that three years together meant..." bolsters this. Three years only means that you have been together for three years. You are not necessarily competing for his affection with someone else
(there is not a limited supply of affection that you dole out) - you are continually renewing your affection for each other (or not, in which case, the relationship falters - regardless of any other people in the mix)
Love is not a goal that is achieved and then set on a shelf as you move on to the next step. Relationships need maintenance and attention if they are going to thrive. You are not in the same relationship you were last year, or the year before, or the year before. It's a living thing that requires tending to - like a plant growing.
Have you two read the "5 love languages book" to help each of you understand what helps the other feel loved and cherished?
Sorry if this is not as clear as I would like - on the way out the door to work. Will check back later.