This lack of emotional range can become a problem because we rely on language to shape our thoughts. If we only have a limited palette to paint with we will struggle to create an exciting picture. Furthermore it makes it difficult to challenge and change emotional states if we are using broad categories. Take stress for example, what does that actually mean? If you were feeling stressed and you wanted to feel less stressed, where would you start? If you can increase your emotional range and dig beyond 'stressed' you may find such emotions as 'threatened, 'frustrated' or 'sensitive'. These are actionable, we know where to start when it comes to these so we are better placed to change them.
If we want to start increasing the emotional range we are aware of and subsequently capable of experiencing then there are several simple things we can do. The first is banning some of non-descriptive 'fake emotion' words we use on a daily basis such as good, fine or OK. These aren't emotions and don't tell us anything so if someone asks you how you are you are banned from using them. Dig deeper and share how you actually feel. Our workplaces and social circles have been sterilised of emotion, we don't feel comfortable to actually 'feel' so have removed the language from our vocabulary. Challenge those around you to join in. Only good can come from it.
Another basic place to start is to have an 'emotion of the week/day' that you can look at and focus on. Explore what emotions are available to you and practice experiencing them. This should be a fun process, setting out to experience joy, nostalgia or serenity each day will help you notice the difference between these and your base emotions and encourage you to explore them more often.
I would also encourage trying an emotional diary, which incidentally is often the recording method for studies in this area, to help you track, record and reflect on your emotional range from day to day. Coupled with the techniques highlighted above it will help you to reinforce any new emotions that you start adding to your range and identify areas of the emotional spectrum where you have limited range.
There is a famous quote, oft attributed to Robin Sharma, that states that the quality of our life is determined by the quality of our thoughts. I agree with this sentiment and would go further to add that the quality of those thoughts is determined by the quality of the language we have available to us. If we are frustrated by our emotional state then a crucial stage in improving this is improving the range of emotions available to us. This allows us to better action and subsequently improve our state, giving us greater control.