121 programs are not difficult or particularly complicated. They do require a particular ethic, which is simply that one must study. I have seen those who showed up for training completely unprepared to study, and who thought that the understanding and knowledge should be infused into them by osmosis.

Considerably more initiative will be required in a shorter period of time, to get through the initial training at an airline, than required for primary training; more personal discipline. Those who arrive expecting the airline to make them a pilot may leave disappointed. Those who show up prepared to take the materials given and put in the effort stand a better chance of success. Hundreds of thousands have done it; there's nothing magic or hard. You can too.

It's not for everyone. There are a lot of jobs out there and airlines are just a piece of the pie. The timbre and pace is not the same at all airlines, either; nor is the culture. There are many other paths in aviation, and some very good ones. Choosing 135 isn't a function of failing out of 121; it's sometimes a matter of preference, sometimes availability, sometimes living locally, sometimes gaining experience, etc. There are some very good 135 jobs out there. There are some that are not so good, either.

Bear in mind that depending on the operation, 135 training isn't a gimme, either. Whatever path you choose, commit to never settling for a minimum standard in your own study, in an employer's offering, in maintenance, schedule or the proverbial quality of life.