Part of what people attribute to "match mentality" is surprisingly often really just technical deficiencies in one's game which cause shots to break down under the stress of unfamiliarity and unpredictability. In a tournament -- maybe in a new building on different tables, against unfamiliar opponents -- unpredictability is maximized. Guys you play who may not look that polished may at least be able to be consistent in their awkwardness; and the things they can do, while maybe limited, are at least reliable, but they may still seem pretty strange to you. They may have limited skills but the ones they have are well honed over time.
Coming back after a long layoff, everything is going to be a bit wobbly when you are under duress, even if your game is built on pretty solid fundamentals. This is not because you are crazy or weak, it is because you have technical issues, probably many of them. In statistical terms, I like to think of it as too many degrees of freedom. It is probably going to take a bit of time to build back the precision you need to play well in close matches. One thing you need to make sure to do is to have a lot of random unpredictable elements built into technical practice. And make sure you are practicing all the elements of the game, not just the parts that are more fun. Of course, a lot of what other people are advising is true too.
My purpose in this comment is to help you realize you are not some sort of "head case" who is mentally weak and that there is a pathway out of your current situation.
It is true that some athletes are unusually great under pressure. A lot of the time, though these are people who fundamentally really really sound.
