The horn i play right now is a holton 378. i have heard that this is a student model and should get a professional horn later on. but what if i want to keep my horn? i've played on some professional horns and they all seem too big and i don't really think i sound that much better on them (except silver ones). what's the difference between these two types of horns and can't i keep playing the horn i have?

Your Holton H378 is a student model. You can keep your horn as long as you want to. You should not go to a 'professional' model yet. To sound good on a professional model you need proper support, lots of air, other stuff. There is nothing wrong with your H378. Student models are made with different qualities and good student models are made so the student can hit notes easier. As you get better and stronger and understand things better you will try some pro model down the road and you will want to switch, but there is no rush for that.

Silver horns will allow your articulations to speak better. The softer yellow brass will absorb quite a bit of your tonguing and possibly some of your tone.