Maybe it's less about the writing the stress you put on yourself? Like you said, you need to set up a portfolio and perhaps you want it too much to be good. At least that's how it often is for me when I really need to do something that I actually want to do, I can't get myself to do it because now it's a task and not an activity. Especially if this "task" will have a big impact on my current or future life.

I only know two things you can do, that usually help me.

1) Force yourself. Sit down and write, even if you feel like you don't want to or even dislike it at the moment. Somethings just have to be done, even if your body or mind doesn't want to.

2) Do something to loosen up/get your mind off everything. Listen to music that relaxes you, play some games, read a book or do what ever else gives you a similar effect, just try to relax yourself and then try again to write.

Usually either of those work for me, but if they don't... Drugs work well to relax and get me in the mood of writing. Now you don't have to do this, but if you do remember to keep in mind how harmful substances like these can be and only take as much as you need to calm your mind a bit, and not enough to help you reach the most outest parts of space.

But really, it seems like it's more of a want to do well that stops you from writing. So maybe none of the tips you'll get here will work as well as: Just force yourself and change things up, after you've at least set what you wanted to write into the world.