What happens when I don't have enough serotonin in my brain synapses?

Serotonin. Some symptoms would include being anxious in typically low stress situations, being impatient, fatigue, can't focus, sugar cravings, aches.
Tricky question. This is a tricky question, as there is no way to measure serotonin in the brain synapses of living humans. Many (not all) antidepressants increase synaptic serotonin, so many people assume depression must be due to "low serotonin." there's no direct evidence for this. Some antidepressants don't act on serotonin, at least directly, & we literally don't know how any antidepressant affects mood.
Hallucinations. Although, lsd increases serotonin excessively and causes hallucinations.
Depression. People who lack seratonin are often depressed and/or anxious. Proper diagnosis can be obtained from a psychologist or psychiatrist evaluation.