I see some answers are more on the discipline side of things, and it is true that your position size should be part of your trading plan.
It is different from your strategy itself - you know it because you read trade your way to financial freedom. The position sizing strategy should tell you how much you will lose at max, on a single trade, per day, week and month. This can be a percent of your equity or an amount in dollar. For the records, a lot of professional traders out there allow themselves to lose 1 to 1.5% of their equity on a single trade, no more. Peter Brant is at 0.6% per trade, yet he has years at +50%, +60%…
Why is that? because when you are down 25%, you need a +33% to get back to break even (and you psychology is likely messed up by the big loss, so that bigger gain is even harder to make!).
There is an excellent book from Tharp, the Guide to position sizing strategies that is also good read. Of course you can only position size accurately if you have a stop, so the rule "no stop entered exactly at the same time of the position - no trade’’ seem extremely mandatory. (and I know apparently a few very successful traders never use stops… but some of them blow out, still). My experience is also that varying bets size depending on our guts is usually resulting in bigger losses. Unless of course you have 3 strategies tested on various and large samples and the expectancy of one is stellar and not the others.
Concerning self sabotage, it is a very good question - and by reading you, it seems to be a self sabotage issue: you do well, with a strategy and product you know, having your risks controlled, and all of a sudden you trade something else, without stops and too big a size. That sound very much like gambling (I am bored and want some fun, I want to make a big win) or self destruction so my recommendation:
1 / stop trading any money for a while - you can still work on your edge/strategies with a demo or simulator
2 / maybe go and take one of van tharp workshop. If too pricey (I cannot afford it) some of his books deal with these, of you can get the sedona method supercourse and deal with self sabotage that way (using it for the last 20 months and lots of non useful feelings can be let go with it)
3 / when you have done enough self work try again real money trading with less size and see how it goes
One saying is that traders pay their education to the market, by losing or taking courses/coaching/mentoring… the later is likely less romantic/exciting/manly than the first but it looks to me more interesting, cheaper and faster.
Good search and self exploration - and don’t be too harsh on yourself, it is not deserved and does not help anyway.