My main advice on this topic would be to try an easier faction. You pretty much jumped right in with a faction like portugal, which can be difficult even if you are experienced. My advice would be to try a larger Catholic faction, like the English. They have a fairly strong position and good unique units once you hit the high period. The Byzantines are also a good choice, unique unit roster, which is pretty much unmatched in early. Also, they have large borders, which means you will get plenty of combat practice.

I wouldn't get discouraged over your recent losses either. I switched to playing RTW for about a month after playing MTW for about a year. I was losing battles left and right when I came back to MTW. The real thing to remember in this game is to not rush off after a routing enemy. It is very possible for them to rally and cut you to pieces as you try and chase them off the field.

You should also definitely read Frogbeastegg's unit guide( the link in sharpshooter's post). It has a lot of useful information on morale, fatigue, formations and all that other stuff that is oh so unimportant in RTW, but could mean death in MTW.

Also, i'm wondering, how were your archers formed up? I know in Rome you don't have to worry about it, but in Medieval, they need to be formed up about two ranks deep, otherwise only the front two rows will shoot.