I don t know what s wring with my car?

So I bought a car it s a 2003 Honda civic LX coupe with a 160k miles I m the 3rd owner. The first day I accidentally filled it with diesel (I didn t know how gas works I ve never owned a car before) so what I did was I called a towing company and towed it again the next day to the mechanic to drain it (note that it was sunday whrn it happened so I had to wait and I didn t turn on my car). So everything was alright but after a day when I was reversing and it was a tilted ground and finally to the flat surface my car wasn t reversing I try to accelarate it but it s still not reversing it was like it was on neutral cause it was moving back and forth and I have to turn it off all the way then turn it on again. I need help:(

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