There's a simple solution to that, don't trade if you can't control your emotion, the most important thing is the implementation of your plan because that would determine the outcome of your trading activity, even how good your plan is, it will only stay in the paper if you can't implement it well.
If you could plan ahead of what you are going to do, then you could make a lot more money.
I personally feel like there are too many people who set out on a road and then switch lanes right in the middle of their path.
If you have a plan that you want to follow and then you do it, it will probably be a good one and if not at least you can see what was wrong on that plan. However when you set out to do one thing and change your idea in the middle and do something wrong then you will obviously know what is wrong which was the idea change, which you will regret but you can't learn from it, all you will realize is that you should not change your idea in the middle.
Just write down what your plan is with that investment, and do not change it, just keep on doing what you set out to do and keep doing that for a long time. Obviously it will be hard to not touch it when it is going down, but that is how you can make a profit, when you do not touch it and wait for it to recover.