He doesn’t seem as interested as I am...

I am a 25-year old woman and finally began dating a guy I have been in love with for five years. It took us a long time to get together because he was dating someone else and simply didn’t see me the same way. It has now been two months since we decided to give this a try, and I am conflicted because he doesn’t seem to show the same interest in me as I do in him. I am always enthusiastic about spending time with him, because I have waited so long for this to happen. He doesn’t show as much excitement, possibly because he looks at this as just another relationship in a long line of them. This makes me sad because I really want something long-lasting with him. How do I get him to change?
You can’t change his attitude and compel him to start looking at this from a long-term perspective when you have only been together for two months. You may have wanted this for a long time, but it will take him some time to get to that same place because, as you yourself point out, he didn’t have the same feelings for you. He wants to be with you, which proves his perspective has changed already. To assume this is not serious for him is doing him a disservice because you have no proof and are simply basing this by comparing his level of enthusiasm with your own. Allow this relationship to breathe a little and evolve in its own time. Give him a chance to get to know you better and be a little more patient. The dynamics between you two will change on their own within a few months.

Should I ask my girlfriend to stop being friends with someone I despise?
That is her prerogative and depends entirely upon how open she is to that conversation, how valid your reasons are, and whether or not she wants to.