Tricky dilema regarding on going freelance work

Hi all,

My first message here so hoping some of you may have some advice!

Im a designer within the construction industry and have been freelancing for one company since last yr, whilst not signing any formal contract, blocks of work extended each time. I was offered a full time position with the company earlier this yr which i politely refused as it was not the right position for me to take on full time. I have on going been working with them 5 days a week as they are very busy, and my current block we agreed was till april end. I have recently been working on projects and getting more involved due to the nature of them, one project will effectively run through rest of this yr, whilst they are busy with other things to fill me in full time in between. The work however is not something i want to be tied into long term,and i had made this clear earlier in the year.i have now been offered potentially bits of work from another client,likely to be a bit hit and miss and sporadic but more along the lines of interesting work i want to be involved in. Here lies the dilema- i dont want to turn down work i feel id be more interested in, but i feel locked into a company for the rest of the year - if i try and extract from this company why would they want me back after deserting them, as i would not wish to burn bridges but have a concern i would be seen as leaving them high and dry if i tried extracting. I understand many on here may have the view that i should be grateful for the long term freelance work,however the reason i wished to stay freelance was not to be pigeon holed into always working on the same things all the time- how do i maintain a good relationship whilst pulling out of a company that could keep me busy 5 days a week for yrs to come?