Welcome to the forum. I've been on Lialda, a sister medication to your mesalazine (octasa), for 5 years now and I have yet to experience a single side effect from it. Octasa is encapsulated in a delayed-release coating, that topically applies the medication to only your large intestine. As a 90% topical medication, the risk of side effects is very, very low and it is well tolerated by most without experiencing side effects. We have some posters here who have been on these medications for decades without troubles. Many of us here have have lived with UC for a long time and are doing just fine. We live mostly normal happy and healthy lives, with successful careers, families, and pursue our dreams and hobbies. I know at first a new diagnosis is quite scary and overwhelming. And it is especially challenging if you struggle with health anxieties and are keen to self-diagnose. Dr Google and self-diagnosis are often wrong, with a very high inaccuracy rate. Best advice there is to unplug, stop the searching, and research and put faith in your doctors to know what's best for you.
I know all-too-well that fears and anxieties aren't easily overcome with just facts and reason though. If it were that easy to say "a small spider cannot harm a 200 pound, 6-foot tall, grown man, there no more fear," then there'd be no arachnophobia in this world and other phobias would suddenly *poof* disappear in a heartbeat. Unfortunately it isn't so. Rather, fears and anxieties are deep animal-brain responses meant to protect us, they're deeply-rooted based on anticipation of danger based on past experience. Generally it takes new positive experience over an extended period of time to conquer fears and anxieties. It is very difficult with lots of ups and downs, but with lots of time, patience, dedication, and help from counselors we can often get through it. There's just absolutely no instant-gratification or quick-and-easy fixes.
I know fears and anxieties are a challenge and many of us here face various health anxieties to different degrees. You mentioned before that you did a lot of self help and seek out counseling, and that might be a good thing to do again. There's a lot of unknowns with a new diagnosis, and at first we fear the worst case scenario (it's in-part human nature to do so). Over time, it does get easier as you begin to know your new illness, see how you respond to treatments and get to know what to expect from your new illness. It's kinda like having an annoying sibling, over time you know what to expect from him/her, anticipate certain behaviors at certain times/events/situations and do the best you can to tolerate him/her with the least amount of trouble that is possible haha.
We do have a Anxieties forum on healingwell, as well. They're a really good group of people who are quite knowledgeable and helpful.
www.healingwell.com/community/default.aspx?f=9