How can a person control fear and be more assertive/aggressive?

You must use your fear to your advantage. You must simply learn to act even when encumbered by it. It takes practice.

Practise.
For now, find small harmless things you’re only a bit afraid of, and do those. Elsewhere I recommended getting into the shower before the water gets warm, throwing out spiders, or googling for photos from high buildings, or whatever creeps you out. I use photos of syringes.
Build up courage that way. Move on to scarier (but still safe) things when you get over your lesser fears. Start small and pile victory upon victory.

Ask your teacher, too. It’s his job as a martial arts teacher to teach you to defend yourself, yes?

Just keep in mind: They can kill me, but they can’t eat me.

This answer may seem off topic, but I started paying closer attention to my own sense of fear and seeing what brought it out and what chilled it out. I started noticing that certain experiences really slammed my sense of self-worth. I also noticed that others could increase my sense of self-worth. So I started setting a target for myself. I assigned points to certain things I could do that helped my self worth (for me it was things like cleaning a room or getting laundry accomplished, nothing major). The point value increased compared to how much I disliked the task.

I decided that I was going to try to earn about 100 SW (self-worth) points every day. If something occured that knocked me back, then I needed to earn that many more points. It took a while to build up to this, but when I regularly practice it, I notice that my fear minimizes drastically. It builds my confidence. When I know that I am actively doing things that make me feel better about me, I have a different sense of being able to stand up for myself. The horrible feeling in my stomach goes away and I find that I stand up straighter.

It’s a matter of training. Repetition of pre-programmed responses to threatening situations. In the Army we called them IADs (Immediate Action Drills); you may be afraid but your body is automatically doing the right thing. Move, shoot, communicate.

A few sessions with a clinical psychologist will help you to cope effectively with inappropriate fear responses and how to distinguish between those and realistic fear responses to objects or situations that put you at risk.

Bottom line – you need to learn to cope with some fears you experience and you need to learn and practice appropriate behaviours including, where appropriate, assertive behaviours.

Aggressive behaviours are rarely appropriate except in combat. I am not qualified to say whether or not they are appropriate in martial arts. The black belts I know demonstrate confident and powerful defensive skills but don’t advocate aggressiveness in dealing with others.