I'm an IBS-D sufferer. Eight Lomotil a day is quite a hefty dosage. I wouldn't be surprised if that was the culprit behind your constipation. When I would plug myself up with too much Lomotil or Immodium, inevitably I'd have a "blowout" a few days later. All that stuff had to come out. Also, if you are that plugged up, you may be experiencing "overflow" diarrhea. Meaning, that the only fecal matter that can get around the impaction is liquid.
I learned that my problems were caused by pelvic floor dyssynergia - due to constantly fearing having an attack and also from chronic pain, I was tensing my pelvic floor to the point where it stopped contracting properly, which just exacerbated my IBS issues. It was causing incomplete evacuation and impaction, which led to terrible almost constant overflow diarrhea.
I took a bunch of dulcolax to clean me out, started laying off the Imodium only taking it as needed, which turned out to be one pill every 2-3 days. I also underwent 5 months of physical therapy for my pelvic floor. It helped tremendously with the pain, urgency, alternating bowel issues, cramping, abdominal pain, etc. I still have rough days here and there, but overall, feel much improved.
Not saying that this is what is going on with you, but that is what caused it with me.
As far as anxiety, I am undergoing CBT and taking Celexa in order to combat the anxiety which is primarily caused by the IBS, which in turn, causes more IBS symptoms. I understand how your anxiety can make the situation worse. Effexor didn't do much for me either in the way of IBS, though it helped my anxiety. I didn't like the side effects, however, so I stopped taking it.
Perhaps talk to your doc about addressing your underlying anxiety first. It seems that the anxiety is being caused by your IBS and is also resulting in making your IBS worse. It's such a vicious cycle, isn't it? Look into meds or therapy to address those situations. It's helping me so far. Meanwhile, eat very carefully - avoid trigger foods, fatty foods, dairy, etc. Try to eat many small meals throughout the day. Exercise and get plenty of sleep. Drink lots of water! That helps with constipation. You could try something mild like Miralax, too, if you continue to feel clogged up. Fiber never helped me, but it helps some. Take Lomotil as needed. Also, heat therapy works wonder for relaxing those abdominal muscles! A heating pad is my best friend :) Or you can simply take a nice hot shower or bath.
Good luck and let me know if you need to talk anymore. I have been through hell and back with IBS and done just about everything there is to address the problems. Hopefully I can commiserate and share some of my thoughts and experiences with you :) Hang in there!
Glad to hear you are doing better. Keep it up. I am suprised to hear that Buspar was your culprit. This is an example of how the meds can be different for each member.
I am sorry I misunderstood. I have IBS-C and none of the meds give me D. I have been on many of the SSRIs/SNRIs.
I am glad your on the mend.