Buddhist teacher Pema Chodron writes in Comfortable with Uncertainty (2002):

Start where you are. This is very important. Tonglen practice (and all meditation practice) is not about later, when you get it all together and you’re this person you really respect. You may be the most violent person in the world – that’s a fine place to start. That’s a very rich place to start – juicy, smelly. You might be the most depressed person in the world, the most addicted person in the world, the most jealous person in the world. You might think that there are no others on the planet who hate themselves as much as you do. All that is a good place to start.

What you do for yourself, any gesture of kindness, any gesture of gentleness, any gesture of honesty and clear seeing toward yourself, will affect how you experience the world. In fact, it will transform how you experience the world. What you do for yourself, you’re doing for others, and what you do for others, you’re doing for yourself. When you exchange yourself for others in the practice of tonglen, it becomes increasingly uncertain what is out there and what is in here.