It seems that they didn't have the guts to talk to me straight in the first place. If they had told me that they want the old bassist to play a few songs, I could have maybe accepted that. But their strategy was to tell me I can't play, and try to leave me out that way. I didn't - naturally - accept that. I proved them wrong, like I do with all the false accusations.
When they couldn't say that I was a bad player anymore, they started to tell how important person this old bassist is etc. etc.
They even said that "it is an honour to play with her".
They suggested that she'd play all the old songs and I'd play the new ones. Which makes six for her and two for me!
I wanted to make a (bad) compromise and suggested that if we'd just play 50-50, but that wasn't alright for them neither. They even wanted to make "a poll" to see who wants me to play and who wants the old bassist! It would be just as democratic as a presidential election in North Korea.
The say I'm a whiner and cause them a lot of trouble. But I guess no bass player would accept such a behaviour. It is gonna be VERY VERY hard to find a bass player who can be put in corner every time the old bassist is around.
Em, gigs come and gigs go. Bands come and bands go. Nothing is permanent.
You've shared snippets with us, and they seem to contain some contradictions, which could be that the rest of the band aren't being straight with you, or that you are still working through things and your perspective is changing. It doesn't really matter. It seems pretty clear to me that they aren't treating you well, and even if they are trying to be kind, it's not succeeding.
First thing, honestly accept they might be unhappy with your playing, and if that is so, there can be all sorts of reasons for that. It might be that you have work to do, or it could just be that they want a different feel. Or the old bassplayer might just be a close friend. McCartney once said at the height of Wings popularity that he missed the Beatles because they just thought so alike they could look at each other and know where a song was going to go. You need to think through this in order to find ways to improve as a player, and it can hurt but it's worth it.
By the way, it may be that the other player IS better than you. No harm in that. Plenty of players better than me, but I still get enough work to have fun

.
Second, if you are a regular band member and there is a question over whether you can even participate in a gig, you have to decide if that is workable. For me it wouldn't be, unless it was someone I really, really wanted to play with desperately.
Third, you have to think about your own development. If they are restricting you to root notes, and you want to do more, would you be better in a band that gives you room to explore your own creative direction? Be aware that a lot of younger players overplay....they think it sounds good to be busy and complex. Many older players come to the perspective that less is more. A simple line well played is more musical than a complex train wreck. (But that still doesn't mean holding down the root). Being honest, your assumption that you must be the better player because you practice a lot and the other bassist doesn't is not logical. If the other player is much more experienced, the fingers remember old skill reasonably quickly after a couple of years break, and their experience may give them all sorts of knowledge about what notes to select, how to behave on stage, etc.
As another poster said, we are strangers on the internet. But if it were me, based on what you have said, and assuming I didn't have a reason to persist, I would just quietly announce I was leaving. I would say that since the other bass player is lined up, I assume it could happen with immediate effect. But if they wanted me to continue for a month or so, I would be willing to do it on a couple of conditions. First, no sharing the stage at all - no "two bass players on stage" stuff, no gigs where players are swapped between songs. Second, no more contributing for common costs (promo, rehearsal studio, demos, etc) but a full quarter share of any gross income the band makes from gigs. To me that is fair. You are probably still going to lose money anyway on petrol, strings, etc.
If they want to discuss reasons for leaving, up to you, I probably wouldn't. But if you do, be totally honest, and get to very specific real issues. You can't work as part of the band unless you have a clear understanding of your role, your rights and your responsibilities.
I would not play games with them or jerk them around, because that way you can look other people in the eye and honestly say you tried to do the right thing in a difficult position (you would be amazed how honesty can communicate itself). And that aside, I would not worry about the rumours. People who know this band will know what they are like.
Start working on a "band wanted" ad for Craiglist or your local music store noticeboard, or start looking for other auditions. And good luck with it. I know it's sucky right now, but it will turn out. Like others have said, bass players are always in demand.