Welcome to the MS Board
Migraines, childhood illnesses, high fever, infections....many things can cause lesions. There are over 400 diseases which can present with MS like symptoms. Your neurologist- is he an MS Specialist? It sounds like he is doing the right things.
If you had Parkinson's, you would have had it present already or at least be in stage 2. ALS is a disease which can be found early. If you have had it for 15 years, you would indeed be a miracle. I think I would set these aside as possibilities. Blood tests will eliminate all of the blood borne possibilities.
You can indeed have another negative LP result. I had an LP in 1982 and again in 2002. Both of mine were negative.
I would set aside your fears and anxiety which can cause MS like symptoms to present. Your appointment is not until the end of July. I would try to write a journal of your symptoms which you have experienced and see if you can find any triggers. These triggers can be weather, temperature, life events.....
Do you have MS? The only answer is that you have a 1:400 chance of having MS. You need to run tests to eliminate everything you do not have. If nothing is found and all possibilities have been eliminated, MS is left as the diagnosis for most people. Go online and print a copy of the Revised McDonald Criteria. Take it with you to the appointment and use it like a checklist or road map. I certainly hope you do not have MS, but if you do, it is not fatal and you can continue your life with a few adjustments as needed.