I find 3 things important: celebrating successes, shore up your mental model of failure, and to try to build mental positive momentum. I will explain.
With regards to positive momentum, it takes a bit of mental energy to start. When I am biking to work or in another time when alone, any thoughts of a defeatist or negative nature I aggressively try to shift to a positive. Example being "man, I'm going to get nothing done today cause I feel tired" can become "I didn't get the best sleep last night but this fresh air is really refreshing, the sky looks really nice today". Once I get started, it's surprising how quickly it sticks and I no longer have to try.
Shoring up my mental model of failure - I guess here if I was thinking about building a side project, instead of "Google has already done it, no point in building it myself" I'd instead focus on "I am going to change my idea a bit and service this niche, and my objective is to learn how to build this, not to win the war against Google". So by picking a personal objective to work towards, personal growth, and in case it does fail, I then still achieved what I wanted.
And lastly, to remind yourself that you achieved a lot as it goes along by celebrating successes. Again, if I am building a side project and I manage to get my first 50 users or something like that, I am going to have a little celebration ritual - doesn't have to be big, maybe it's as simple as "awesome, I am going to celebrate this by going to my favourite bakery and getting my favourite croissant for breakfast this morning" or something like that. It helps to do this to maintain positivity throughout.
Anyways, that's what helps me personally.