I sympathize a lot, because I was afraid of those two diseases as well (but not at the same time).
With the ALS, I thought I might have it because neurologists noticed heightened reflexes in my legs, and a little bit in my arms as well. This lead me to read up on all kind of terrible motoneuron disease stuff, and ALS of course being the worst of it all. Add to that the fact that I'm generally pretty weak physically and my muscles tire easily, and .. yeah.
What helped me:
- Seriously thinking about the likelihood. ALS is a rare disease, and mostly occurs in people over the age of 40 or even 60. If you're below that age, it's very unlikely for you to have ALS (though not impossible, true. I know THAT line of thought.)
- The main symptom in ALS is serious muscle weakness, that usually manifests at first in either the lower legs, feet, fingers, or the muscles that allow you to speak and swallow. Almost all patients with ALS, the first symptoms they noticed is not being able to walk properly anymore, slipping more frequently, not being able to type or write correctly anymore, everything feeling a lot heavier when they pick it up, or problems with talking or swallowing.
- Test yourself: can you stand on your toes? Can you walk around on your toes? Can you stand on your heels? Can you walk around on your heels? It doesn't have to be graceful. If you had ALS, it'd be very likely you just couldn't do it at all, not even a bit.
- Muscle twitching is a VERY common thing and in 99,999% of cases ALS is not the cause. Also, muscle twitching is something that can be VERY easily influenced by anxiety. When I was more or less convinced that I had ALS, I was literally shaking all over for days, I felt that I was too weak to carry even easy stuff, and I felt like my legs were too weak to walk stairs. This was all psychosomatic, I do not have any of these symptoms anymore.
- If you're still worried, try if you can have your blood tested for Creatine Kinase (CK). This is a parameter that will rise to very high levels when muscles are being destroyed. ALS destroys muscles, so in patients with ALS, CK will generally be elevated. If yours is normal, it's almost impossible for you to have ALS.
All of that helped me, hope it can help you too.
As for Colon Cancer, I was very afraid of that too, after I once had *terrible* bowel pain and then quite a bit of light red blood in my stool. That happend only once, but got me in a total panic.
What I did was do a lot of these tests for occult blood in the stool. I did one every two days, for 2 weeks or so. They were all negative. To be even surer, I also had a coloscopy done, and everything was fine. This helped me get over that fear, so I can recommend it -- although the coloscopy might really not be necessary, it's just if you have to be REALLY sure.
Be strong :)