Lyme bacteria can mess with neurotransmitters, which can affect mood. The Lyme can also mess with hormones, which also can mess with neurotransmitters (hence PMS). And my understanding is that some of the coinfections can also do this (Bartonella, Babesia).
And yes, chronic illness of any variety can contribute to this. I think it is important to allow yourself to feel the emotions instead of trying to resist them. And then it is also important to have activities, things, people in your life who can come along side you and help you through it when you are struggling to stay positive. And I think this support forum can play a big role in this for those who are comfortable sharing "in public." So take heart, you're not alone in this.
Chronic Lyme Disease, Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication/Chemical Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, Pancytopenia, chronic malabsorption/malnutrition, etc.; G-Tube; Currently TPN-dependent.
Meds: Zofran, Pulmicort, Heparin (to flush PICC line), Claritin, Colloidal Silver (used topically), IV Milk Thistle, probiotics.