Whether to get different letter of recommendation when supervisor's letter is bad? I've graduated from my previous grad school (psychology) for 2 years and now seriously considering to apply for another graduate school (or professional school at master's level), computer science in particular, for next year's entry. That was also the major of my college degree. I will be applying several Canadian schools now and hope I will be able to start a new life there.

I was extremely uncomfortable when I just got into the psychology graduate school four years ago that I didn't do well at first. Not only due to English being my second language, but also I had absolutely no background in psychology, plus the massive amount of reading filled with weird words drove me absolutely crazy. For this reason, my major advisor had me finish with a master's degree instead of PhD, which I was originally enrolled in. However, he was extremely helpful when I was doing my master's graduation project and thesis. He was helping me day and night with my work that I believe nobody else could ever help his student this much. I ended up finishing my master's level of psychology very well and left with that degree.

When I got in touch with him recently and asked him to write a letter for my next school, he said 


  "I will mention that at first you didn't turn out to work very quickly that you had trouble to get what I was asking done. So I had you finish with a master's degree instead of PhD. However, later you learned to work much harder and your final project and program worked very well. If you think the above contents suit you, let me know where I should send the letter."


I don't know for sure if my feeling is correct, but I feel like I will get 100% rejection even from mediocre schools I'm going to apply, if those words appear on the LOR. 

Question: should I switch letter?

Concern: I didn't ever think one day in the future I will need to go back to school when I was still doing the psychology program. Thus I was't enthusiastic at all about building my connection with professors so that two other members of my thesis defense committee really don't know me very well.

This is probably the last chance for me to change my fate, I don't want to lose it. 

Suggestions please, and any help is much appreciated.
As I see it, the adviser's reply is in order. He openly stated what the contents would be and emphasized the option to choose another letter, if it suits you more, without any implication that that act could be taken in a negative connotation. 

Further, the contents seem to align with the story you told here, i.e. you started a PhD, but finished with a master's degree. He states you strengths and weaknesses, which is to be expected. What is more important, he intends to accentuate you growth during your studies. This is a powerful statement from his side. As I see it, his response is objective.

Additionally, given the fact that your adviser was extremely helpful during your graduation ("day and night"), which I understand him to have taken on extra effort to see you graduate, I see no evidence that he would sabotage your attempts.

My opinion is that you should use this adviser as reference, as I seriously doubt that you could get a stronger letter from someone else (unless you neglected to mention that you had other collaborations where you fared better). Although other persons may not say anything bad about you, they can probably say also nothing positive, and luke-warm letters are not good. I would always opt for a strong letter, from someone who knows you and can attest to your journey through grad school. It is nothing unusual to have a bad start, but the fact that you saw it through and earned praise towards the end is a very good thing.

Finally, you should probably menage your expectations. You shouldn't expect that you'll find anyone willing to lie for you in a letter of recommendation. From your post, your adviser is a good guy, so, again, I think you have the best chances going with him. Or, put another way, your adviser said nothing that wasn't true, so this truth might or might not hinder your way into some institution, but that state was not induced by your adviser, he simply reports what he observed.