It is much easier to "do" your new character for short set periods at first…a couple of hours…then a day. A time period with a beginning and an end.
Like actors do in a play or film. They identify with the character for the duration of the production, then have no trouble dropping that character and taking on another. They give over the executive/directorial function. When the director says "its a wrap" they can move on...the production is over. Drop one character and move on to the next.
This does not happen in "real" life. There is no end to the identification with the character you are more or less stuck with…your ego/personality. Hence, if you try to change there is baggage that is carried around like an unwanted sack of shit. There is nobody outside…a director…to call action, cut and finally wrap, so you can drop the sack of shit and move on. The conditioning kicks in. But you can drop the conditioning for well defined short periods with less anxiety.
You have to learn to do it yourself…be your own director. Drop the old character, at first, for only a day. Take the risk of "doing" the new character for defined short periods at first…without anxiety. Make the transition gradually.
This effect can often be noticed on vacations. Especially in a foreign country. You don't "feel like yourself". You can drop the old you for the duration of the vacation, then when you return home the sack of shit is there again.