So, an amazing guy who ive liked for months and months is finally going to ask me out.. the problam is, i always lose interest once i get the guy.. how can i not lose interest in this guy?

The question that pops to mind is "what are you looking for?". If you're looking for a stable relationship, with realistic expectations, you should probably allow for the fact that interest goes up and down in a normal relationship, not every day is buzzing with excitement.

If you're looking to feel the thrill of new love, the only way you're going to sustain that is to keep trading them in all the time... inevitably, that thrill gives way to the practical issues of making a relationship work, and hopefully the development of a more stable kind of love that doesn't always sweep you off your feet, but provides other benefits.

If you're looking to have your ego pumped by having a guy pursue you, then I think again you'll need to trade them in on a regular basis.

So some introspection is called for, I'd say.

hmm how do I explain this...
You can't make yourself not lose interest, you don't have mind powers to zap your own mind or anything like that of course. The only solution I can see is for you to sit down and figure out what is causing this. Are you dating really boring boys? Are you merely interested in the chase and the rush of being pursued? What makes you lose interest? Are you looking for love or just love to be loved?

I think if you can figure that out then you can start to fix the issue..because if you really like someone you never lose interest if they are an interesting person..or do you just like guys to want you and once the chase is over you discover you never wanted them at all, you just wanted them to want you.

OR maybe you just aren't a good listener? Maybe read a few books on developing better communication skills?